We sailed on the new ship Magnifica for seven nights beginning on March 26, 2010. This was our first cruise and we brought our kids aged 14 and 17 who did indeed sail for free.(there was only a minor service charge for them of about 25 Euro each total, not per day)
After having read so many negative reviews about MSC we were somewhat scared of what to expect, but everything was excellent. I would say the only negative thing we experienced was trying to eat at the buffet which was very crowded, but we simply avoided frustration by either eating at the restaurant (free, not one of the pay extra ones),or by going to the buffet 20-30 minutes before it closed.
The ship is very nice and reminded me of a Las Vegas hotel-lots of mirrors and brass and lounges/bars with bright colours everywhere. There were two pools, but one was quite cool and the other was full of kids most of the time. The hot tubs were also always full of people, but we are not big on hot tubs anyway.