Reviews starting to come in:
Rotten Tomatoes: 100%
Metacritic: 66%
A high Rotten Tomatoes rating but slightly positive Metacritic rating indicates that most critics consistently liked it, but only liked it a little. The general consensus I'm getting is that the plot structure is kind of a confused mess, and the filmmakers don't worry too much about whether or not the movie makes sense, which is entirely appropriate when the main character is insane. One critic described it as a very violent, very vulgar version of Bugs Bunny, which is also appropriate for the source material.
It sounds like FOX allowed the filmmakers to take a lot more risks, which is highly unusual when it comes to FOX and anything involving one of their precious characters from the X-Men franchise.