We have been to Tulum before and stayed just a short way away from Ahau so we already knew to expect the amazing beach.
We saw Ahau being built on a previous trip and decided we would stay December 2012.

We stayed (for 13 nights) in a Junior Suite on the beach (lower level) as we were with our 3 year old. The room was, I have to say, probably the best bit for me. Clean, bright, close to the sea, really beautiful (though could have done with more storage space).
There is a good sized veranda which worked well for us as our son can play whilst we relax.
The rooms are at the upper end of what you might expect to pay at a good place in Tulum so your expectations are set quite high.
In terms of the room, I'd give 10 out of 10. Issues that niggled a little were:
- Plates being left for 24 hours after room service and the dogs/cats/wasps being attracted to it!
- No kids menu; one day we were charged US$ 31 for a childs fish and chips
- Not a great choice of wines :-( - you'd expect in a place like this, they would have a couple of nice ones (as Villa Estrellas does)
However, the staff were great and they did adjust the price of the kids meals when we drew their attention to it.
They seem to really want to please and make your trip the best so they have to be commended on that.
Special mention to Jack and Edgar - they were very nice and welcoming.
Overall, the food is pretty pricey and compared to some of the other restaurants in Tulum, this is not always completely justifiable. It was though, on the whole, good quality and pretty tasty.

There was some amount of repairs during our stay (ie. to water pipes) but this was done very quickly and didn't really affect us.

To conclude, I'd recommend Ahau though there might be some slightly better options in Tulum if you are with small (ish) children...

Its as in your dreams: Blue Sky, Green Ocean, Palms and white Sand. And this Hotel fits into this dream! Design following nature. Luxury if you want and can affordable it( 250 bucks and higher) but also Bali huts tiny and romantic( for around 80 bucks) very friendly staff and good for breakfast and dinner. They offer Yoga Lessons and paddle tours on Surf boards.
After a week its difficult to leave that wonderful place.

I stayed here 2 of my 8 nights in Tulum and it was divine! I believe the comfy bed in Master Suite #1 is the closest bed to the waters edge in the whole town! The owner David was very helpful and personable, as was whole staff. They genuinely care about their guests. Nice to stroll to a few shops and chill in the hammock. Pets are allowed and the crowd is VERY colorful here. They have yoga and activities (but I didn't make any). Definitely a great choice if you want very high standards in Tulum (which can range from hostels to 5 star) - almost ALL cool Cabanas. Now it's one of my favorite places in the world!

We stayed at Ahau Tulum from 12/17-12/19/12, and next time plan on staying for much longer! It is a really beautiful, serene way to change things up. We live in a large city, and to be able to go somewhere so conscious of its gorgeous environmental surroundings and just slow down for a little while was fantastic. Prior to Ahau we stayed at a smaller all-inclusive, and this also was a great change of pace. Can't overstate how cool Ahau's construction and layout is.

We reserved one of the little Bali huts, which comes with a very comfortable bed and access to the garden shower and little eco-toilet (not a big deal at all if this causes you unease - vastly better than a latrine or port-a-john). Only minor issue with the Bali huts is that they do not offer much privacy. You share the hut and shower/toilet with another couple (we had a mother and daughter in the room below us). We had no problems with our neighbors, but just expect a communal atmosphere with the Bali huts. I don't think this is the case with the other two arrangements you can purchase.

We spent most of our time while at the hotel in the common room (which is awesome, beautifully built), in their restaurant (YUM, healthy and delicious) and on their gorgeous beach.

The staff is extremely helpful and very nice. We didn't want for anything. Only thing we'll do differently next time is splurge on one of the Master Suites - once you see them you'll understand.

Ahau is a great value, nestled in a cute community full of ex-pats and some hippies, with great access to the Sian Ka'an Biosphere. Only slight warning is if that new-agey vibe is a real turnoff for you, Tulum in general may not be your place to stay. It is not overwhelming but if that's a negative for you, it's definitely there. We enjoyed the earthy, chill atmosphere, the delicious food, and the tremendous beauty of the place. We'll be back! Thanks Ahau!