Went to the colosseum not on an organized tour. Guides abound in front of the place and many have official looking credential tags around their necks. They offer a "package deal " of the colosseum and the forum all for a little more than the price of admission to only one of the attractions. Beware!!! The second guide never shows and you have no ticket to get into the second attraction! This is a racket that has been going on for some time here and people just don't know. Make sure you get your ticket. Don't go with an "errant" guide.

The Colosseum is obviously a must when visiting Italy. The ticket (in June 09) is 12 euros per person which also gives you entry to Palatino in the Roman Forum. Buy the ticket at Palatino where there is no queue, the queues are terrible at the Colosseum.
The forum is free, but it is very interesting visiting teh Palatino.
In all cases this is really not disabled friendly. The forum is flat, but like most of rome uneven. Palatino has many steep steps as does the Colosseum.
All highly recommended.

We arrived at the Colosseum 5 minutes before opening and the lines were already rather long. After going through the mandatory security check, we used our Roma Pass to bypass the line which we felt was very convenient. We listened to the Rick Steves' audio tour (http://www.ricksteves.com/news/travelnews/0602/italy_downloads.htm) and while it was informative we felt it was a bit corny. Overall we enjoyed the Colosseum but felt that the stage, gift shops, sound equipment, etc. took away from the historic feel of the site. We purchased our Roma Pass at the tourist information booth at the FCO airport which we felt was quite convenient. Visit (http://www.roninrome.com/2009/04/27/should-i-buy-a-roma-pass/) for additional information on the Roma Pass.

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.