Just got back from 7 days at this resort. A la carte restaurants were excellent and theme night provided interesting variety for the buffet. We were impressed with the friendliness of staff and with how clean the resort is. The beach was nice. I went snorkling almost every day from the beach and saw turtles every time. I have been to several all inclusive resorts and this was definately one of the best.

We have been to the Sirenis a few times, and it never disappoints us. We traveled twice as a couple, once with our three children. 17, 18 and 19 (trip in June 2011), and then again as a couple last year. We are planning another trip this year, and it looks like the prices from Vancouver, Canada will be a bit lower this June 2013. Travel is more expensive than usual at the moment, regardless of resort or location.
Things to do:
If you like to snorkel, the Sirenis is a great destination. Akumal Beach is a short bus ride (20-30 pesos) away. Taxis, are of course, always available for the timid or lazy. Snorkeling at the resort can be amazing, especially for those who are new to snorkelling. Large, colourful fish can be found in water less than knee deep, at the resort's beach. Limestone rocks and crushed coral beach sand make the beach at the resort different than some others. This is not a mile-long flat expanse of beachfront. Rather, it is a couple of smaller beaches, protected by limestone formations and reef. (And a lovely Mayan ruin). Bring your own snorkel gear.
Playa Del Carmen is a delightful little village with traditional local, and "American" Shopping. Some stores offer favourable rates for American money, and some are better with Pesos. Canadian money is best exchanged in your airport or at a currency exchange in Playa Del Carmen. Don't go to the first exchange booths you see, the rates usually get better if you continue to wander past the first shops. The resorts (Sirenis included) exchange C$ and US$ for pesos, but their rates aren't as good. Happy Hour in "Playa" will get you 4 beer for 6$ or less, if you get thirsty while shopping. While we are on the topic of beer, the Sirenis resort serves Corona on tap. While it isn't my favourite beer of all time, it is better than the brands that many other resorts serve on tap. This is important if you are going to have several during your stay. Bad beer gets tiring very quickly for us, but the Corona (with lime for me) works okay in the tropics. As for the shopping, your 5 star resort wristband will make it hard to haggle for souveniers, so wear a long sleeve shirt to hide your obvious wealth status wristband, or, like us, happily pay a bit too much.
Chitzen Itza is one of the biggest attractions, and should be on your bucket list, if it isn't already. If you have some young and active explore types, then go to Koba, I have gone three times and having a snack atop the pyramid at Koba will fill you with awe. You can have the bell Captain arrange to hire a taxi and driver for the day for you, and it is much, much better than one of the "packaged tours". Stop at Tulum ruins and Tulm beach on your way back from Coba and you get three stops in one day. The taxi driver will happily take you anywhere you want to go, and if you are travel savvy and speak Spanish, you can negotiate a better rate for yourself, but the Bell Captain deal will ensure that no sketchy things will happen to you, as the taxi's rely on good will with the resorts.
Like many resorts, the buffet food is not 5 star, but it is, at times excellent, and seldom bad. There are so many choices in the buffet selections that everyone manages to find something. If you can talk your concierge into a extra theme restaurant, these are better, and some are excellent. I think you get two (maybe three?) per week. We have managed to get longer than a week in our recent visits, and this gets you more "a la carte's) We love the Italian and the Mexican, and we had a very good meal at the steakhouse, the one time we went to it. The Japanese does a "Kobe's" kind of meal and show. Young and old alike will enjoy the food (very fresh and good variety) and the entertainment from the chef. My partner is a vegetarian, and we have been extremely pleased with the effort that the restaurants will make, to provide her with something special. Having a vegetarian member in our group will explain why we have been to the steakhouse only once :)
If you choose to drink while you are there, I highly recommend the tequila bar that is in the downstairs south side of the lobby. The caliber of the employees that the Sirenis hires and retains is very, very good. Please tip well, and tell them I say "Hi".
Tipping:
Although tipping is "included" in your resort stay, please be generous with the staff that make your stay special. Very little of the tourist dollars make it onto their paycheques. The fridge is stocked by a different person, than the room cleaning staff, so a buck or two in the fridge helps. Like cruise ships and other resorts, the housekeeping staff at the Sirenis leave "towel buddies". Silly as it is, we look forward to the latest daily creation. We have had some amazing creations. Perhaps it helps that we leave our room staff's tips on top of the towel creation. Safes that you program yourself are included in every room. Use them for your valuables every time you travel to any location, in any counrty as it helps keep the career thieves out of the resorts. We have never, ever had anything go missing in the Sirenis, (and I have left a few easy to steal things lying around, as a way to check) In past trips elsewhere, I have had a credit card skimmed near LAX, and have had my room broken into twice in LA near Disneyland, so if you are worried about crime, avoid Los Angeles. Go to the Sirenis instead.

This is a really nice resort and the design of hotel is quite modern and obviously was much nicer when it was first built.
I went down with my wife, two elderly parents and 2 year old child for two weeks - so that's a risk in itself wherever you go.
Rooms were spacious and clean and we had no issues.
This is a big resort and so I had to rely on the golf carts to get my parents to the restaurants and beach and not so readily available due to the busy time of year and definitely not for lack of effort. The guys running the carts were very generous and were more than willing to help.

Food was fairly good especially the grilled fish or meats. Beach food was a nice treat as well. Italian and Japenese restaurnats are good as well.

Beach is visually beautiful but beware of coral at any point in the sand cause I stubbed my toes at least 4 times during my trip and it was quite painful. Lots of rocks when you enter the water but smooth sand after that.

Pools are very nice but activities are mediocre at best. Not that engaging and at one event they had a soccer competition with the nets set up on either end of the pool, right in front of the people so my poor dad ended up getting a ball right in the eye and just an apology. A few other people ended up with balls in their back and I'm sure they weren't too happy either.
Lazy river was a treat and great idea.

Rooms were extremely noisy and considering we were positioned close to the entrance, we heard all the drunks screaming in the middle of the night. Hotel should consider putting up signs just to remind these incosiderate low-lives that they're not at home. Probably one of the noisiest hotels I've ever been at.

Restaurant crew overall was very good and especially the Bayou by the pool. Thanks Francisco for the great service.

Shows were mediocre and not much effort from the crew except for the Circus theme and traditional Mexican show which was not part of the crew show.

Being a sceptic I realised we were travelling to a third world country and wasn't expecting too much. I was over impressed. Amazing place, enormous, friendly, accommodating, futuristic, beautiful and the food was fantastico!! The things that I wished I knew was the fact that February/ March is the middle of winter, even in Mexico, so the 21C weather reports for the evenings were a lot colder with the breeze from the sea, 12/14C a few more long sleeved clothes would have been good. There were hardly any bugs, insects, mozzies, considering you're in the middle of a jungle. I only used repellent for the first couple of days and didn't get any bites in the whole two weeks. The Mexican sun is lethal, I don't know why but it burnt more than anywhere else I've been in the world so high protection worked. A 100 $1 bills stands you in good stead for tipping, or $20 Mexican peso. The Collectivo bus service was brilliant, out on the highway you could get up and down the coast for under £2 anywhere. It was safe and the Mexican people were really helpful. The medical room on the complex was brilliant, the gym, empty and adequate air con and a good supply of free weights. There was some great bargains in Player del Carmen and whatever you do if you like beaches, get down to Akumal beach. cost £1 on the collective bus, spacious, palm trees, turtles, stingrays and over 70 different fish. I would have taken my own snorkel, mask and flippers in hindsight but all in, it was 5 star, beautiful people, wonderful beaches, fantastic sun, magnificent hotel.