We went to
Legoland in California in March -- it is much different than Disney or Universal. It is much more geared to young kids -- boys mostly. As an adult, it was neat to go once, and only once, to see the miniature cities -- they had replicas of Las Vegas, New York and New Orleans. We were there a month too early to see the Star Wars area, but they were setting it up and it looked pretty cool.
However, the rides were very kiddy -- a step up from fair rides, and nowhere near the ambiance of Disney rides. None of the testosterone of Universal, either. The theme park didn't have that "Disney" escapism feel to it either.
However, there were lots of places to build legos, etc. My 8 year old son is obsessed with Legos. My kids had a great time, but once they are through with Legos themselves, there will be no reason to go back. We'll probably spend a day at the new one in Florida when we are at Disney next June, but it will be at the expense of Universal which we will put off for another year while my kids get taller.
In short, if you kids like Legos, it's a must-see once. Otherwise, no.