Good but ... the worst from LEGO series creators.
From other reviews I read that LEGO Harry Potter takes a lot of time to make 100% and it has a lot of quests. But comparing to other titles from series is almost the weakest.
It took me 19 hours to do all story mode levels and 4 bonus stages and during levels I was really collecting everything, I mean everything. We'll see how much it'll take to do 100%.
The game is very easy. But it's for kids (from whom I "borrowed" it :) Not so much humor as in other titles, not so many thing you can do co-op. Sometimes the knowledge of Harry Potter books is necessary to go further.
So I can say it's best for Saturday or Sunday afternoon when it's raining outside. To play without stress not as in LEGO Star Wars II: The Original Trilogy - challenging Death Star level without loosing life :)
Yes, even games for kids are getting easier and easier.