Yes, Bug is based on a play. I just saw the film version at FantasticFest. Michael Shannon, who plays the lead opposite Ashley Judd was there for a Q+A after the screening and he shed some light on the project for us. He played the same role in the stage production.
The film was quite good. You could certainly tell it was based on a play - the dialogue had that certain ring to it. Very crisply and smartly written. Some incredibly funny moments.
Ashley did a wonderful job - I think it's one of her best performances. Michael was also top notch - really going through a range - his character had a wonderful arc, as did the story. It was engaging from the opening frame - wonderful cinematography. Not every last detail was hashed and rehashed - allowing the audience to participate a bit (but not so much that it was distracting). It's funny, the more I write this, the more I think I enjoyed it. I'll have to catch it again!