I think that the idea of a hotel for monsters is a great one. Who knows what quirky, funny and slightly scary (in a childish kind of way) situations may come up? It's also a good opportunity to include in-jokes for those adults in the audience who grew up with films about Frankenstein, Dracula et al.
Alas, the execution isn't half as good as the idea. The film follows a very predictable path and nothing very original comes up. The idea of a place full of monsters is mostly a wasted opportunity as the monsters are never really given the chance to be very monster-y. You could write the same story with almost any other old characters.
That said, the characters are nicely drawn, the film has a couple of good laughs and Frankenstein is particularly adorable with his large extremities, loose head and his cool bass guitar playing.
One more thing: I think the director has a really cool, appropriate name for a monster film, Genndy Tartakovsky.
Overall, it's a decent film which will probably keep the young ones in the audience entertained but it's also a missed opportunity to make some something really special.