My overall impression of Identity Thief is that the story is better than the movie. That said Melissa McCarthy is one of the most talented comedic actresses since Lucille Ball (IMHO). I'm not sure who would be strong enough to play her straight man. Jason Bateman gave it a good college try, but just came up short. The casting of him to play the role of (the real) Sandy Patterson is the only weak part of the movie. But, that puts the burden on McCarthy to carry the load (no pun intended).
I have not enjoyed so much physical comedy in a while. And, I am not talking as in The Hangover (1) which was excellent, but more like Lucy and Ethel in the candy factory. McCarthy's timing is uncanny. Her character is quick to act. She doesn't miss any opportunity to swindle, con, dodge or fiddle an unsuspecting victim. Her physical size does not hinder her from much. Instead Sandy aka Julie, Diana, or Marji, the chameleon uses it as a cloak of innocence.
Don't let the slow start of the movie fool you. Once McCarthy's character explodes on the scene, the rest is a rodeo complete with the clown. Even when the movie slows (and there are some sluggish scenes) there is a sweet story weaving its way through the background. You're not sure you want to kill her or just ride along with her for the sheer madness of what she attracts. And, Bateman's character, whose life up until this time, has been one of milktoast and number crunching, is so out of his comfort zone he is only driven by his desire to get his life back for the sake of his family.
After seeing a glimpse of McCarthy's talent in Bridesmaids, this is a vehicle for her to shine. And, she is brilliant. I recommend the movie.