Funny enough, I just watched this movie a couple weeks ago. I was watching it with friends as we wanted to watch some Robin Williams movies to remember him.
And honestly, I think this is a great black comedy. Robin is just as funny and energetic as ever. Ed Norton is great as the naive dork who got thrown into the surprisingly seedy world of children's entertainment. And I love that the Irish Mob is among the "Good Guys" in this movie. Yes, you read that right, the Irish Mob are good guys. I'm half surprised Jim didn't mention that.
However, I might know part of the reason this movie was not well received. This was one of the first movies Robbin Williams did that were rather dark. Yes, it came out the same year as Insomnia and One Hour Photo, but this one was also a comedy. Robbin Williams was generally well known for family friendly comedy. That might have put people off of this movie just a little.
On an unrelated note, I just went over his IMDb page. He was prolific and had a much more varied career than I realized.