Yes, I have been on 2 actually, a 3 day in 2001 and 7 day in 2003. Right after I went on the Magic in 2003, they changed the teen place. It used to be Common Grounds, a coffee shop-like place. Now it's The Stack, but I don't know what it's like.
The activities they have for teens are for the most part are pretty cool.
The best part is having friends to hang out with during your cruise. We saw a few movies together in the movie theater on the ship, or sometimes we would just hang out on deck (it's so cool to be up on deck 10 late at night). I still keep in touch with a few people that I met through AIM. Hope you enjoy your cruise!