The Colosseum is certainly a landmark of Rome. Entrance costs €12 (inclusive of Foro Romano and Palatine Hill). I visited it on the 2nd Sunday in May 2009 and surprisingly, the queues were not long (max 5 minutes) in the morning (9am) and late afternoon (3pm). The audio guide costs €4.50. I would recommend you to bring earphones along so that you can plug them into the audio guide and hear better (or even share with your partner). However, when inside the Colosseum bookshop, I saw a Colosseum guide on sale for €4, which I think is a better alternative compared to the audio guide.
One disappointment is that you only get to walk around the arena. I was hoping for some underground passages that we get to walk through.

1) Licensed tour guides with clearly-displayed badges will give you an informative and interesting tour. You will bypass the line entirely, not waiting more than it takes to round up 10 or so people. I highly suggest going on a tour rather than staring into a guidebook because you can soak up your surroundings while appreciating its significance.

2) Steer clear of the gladiator-costumed men. When you see them smiling with a tourist, what you don't know is 5 minutes from now when you're off somewhere else, that tourist is miserable and trying to keep from being completely ripped off. Unfortunately there are a lot of people in this trade in Italy, and as long as there's willing participants, it's not going away anytime soon.

3) If you are going to use the Metro at all in Rome, there is a stop here. (In general, Rome's subway serves mainly residents, as the lines do not cross into tourist zones because of all the archeological interest).

4) At the end of your visit, check out Palatine Hill next door, the 'originial Beverly Hills' of ancient Rome.

5) If you're dying for some lunch, just keep this in mind: if you are walking around the big tourist areas, and spot a huge menu in 5 different languages guess what: YOU WILL NOT BE EATING GOOD ITALIAN...it will be crappy imitation Italian that no respectable Roman would be caught dead eating. So just in general, the further you walk from the white tennis shoes and fanny packs, the more likely your meal will be delicious and memorable.

The Colosseum has been there for over a thousand years, so I guess there's nothing new for me to say here. Whether you'll enjoy it or not, depends a lot on your expectations. For myself, I would say that I expected to be awed, and wasn't. It is definetly a must-see in Rome though, so no matter what you have heard, you should go to make your own conclusions.

The one most important tip here is for you not to stand in the huge line that curls around the Colosseum. The got the best tip I could possibly find here in TripAdvisor before my trip, and would like to pass it on. Tickets for the Colosseum include visits to the Palatino Hill ruins right next to it, and most importantly: VICE-VERSA! So make your way to the Palatino Hills ticket office, where there will be about 10-15 people in line (as opposed to about 500), buy your tickets, and then go back and walk right into the Colosseum without standing in line.

Another important tip, which I failed to follow: bring a bottle of water - especially in the summer. You'll be doing a lot of walking through the ruins, and it's good to be hydrated at all times.

For a site to see, it is worth the visit.
The tips I can give is:
Get an audio guide verse a Video guide. My reasons being is that the video guide they give you re-used ear buds, and you do not know if they have been cleaned as they are not sani packaged. The volume is adjustable but it does not get to a high volume. The video guide is basically like watching a discovery channel show.

I recommend the Audio verses a human for a tour guide. The audio you hold up to your ear.

We also went in the winter and the lines were short. Out side the Colosseum there were many tour guides- with no company logos (I assume college students) that offered a tour where you can cut in front of all the line. However you have to pay for the guide + your admission fees. It would be worth it if the lines were long, as they where when I was there in the summer the time before. They also offer a guide to the Forum across the street. It is not worth it in the winter- Id just get an audio guide.