Definitely rent it. You prob wouldn't want to buy it unless it was less than 10 dollars and you just have to own every Jim Carrey movie.
It's funny: not-laugh-until-your-stomach-hurts (Dumb & Dumber, Me Myself & Irene), but MUCH better than Bruce Almighty. Has some really funny gags/scenes (the job interview, his TV appearance, his first day laying around the house, the opening scene, the last scene...)
I too saw this movie having no clue what it was about (the poster suggested it was something like Catch Me If You Can)...and I was pleasantly surprised by how funny it was!
But of course, Chip Douglas singing "Somebody to Love" pwns all.