I will agree that the premise of this movie - a man secretly hiding in a house without anyone detecting him - is a bit hard to swallow. But I found this movie strangely compelling. One reason is that the "bad guy" isn't without sympathy. We see that he is still insane, but that's no fault of his, and that he struggles very hard not to do anything bad. (Busey gives a great performance, by the way.) Some events in the movie, like when he starts to introduce himself incognito to the family, are unexpected and intriguing.
However, starting about 20 minutes before the end, we see the movie start to fall apart, and the climax is way too familiar and expected - it's almost like the screenwriter refused to change from a typical cop-out ending.
Otherwise, not bad.