Kids had fun! Just left the Sapphire Princess (Feb 2011) where we left from LA to the Mexican Rivera. We have a 7 and a 10 year old. The Kid area was well staffed, had a ton of games organized, and plenty of video game areas. My kids gave this area an excellent rating.
I loved the ship. Medium to large, and I kept getting lost/confused. My wife speculated that it was my sea sickness patch (which worked quite well). The Food was too good. I could have eaten every meal at the buffet but dinners were special in the main dining room where they treated us like royalty, and superior choices.
P Vallarta was usual. Mazatlan was my favorite site. We disembarked and were swamped with people selling time shares. We picked one and agreed to do the 90 minute presentation. It was well worth it; not because we wanted a time share (we did not buy), but they paid for our cab fee to and from, we got to stay at the destination all day and eat, drank, and use all the amenities for free. Our bill came to over $150 (did I mention free beer and food) but we paid $0. I wish we would have known this earlier and gotten off the ship earlier.
The ship staff were great, place was clean, and activities abound. I heard one critic say that this was an older crowd. We are early 40's and did notice you had to hunt for the "fun crowd." I would think a party ship would be a cheaper ship like Carnival. I would love to cruise with this line again.