It's not really supposed to be a comedy?
Has some very very dark, subtle comedy in there, but it's not really a comedy overall and if you watch it expecting it to be such, you're obviously gonna be dissapointed.
I personally love the film. Quite unconventional for a Tarantino as well. Far talkier and slower with less "LOOK AT ME IM A STYLISH DIALOGUE SEQUENCE BETWEEN A COUPLE OF GANGSTERS HURRRRR" shit. This flick shows that Tarantino's a master of tension when he tries as well.
A 9/10 for me, and I now eagerly await whatever he does next. I'd totally written him off before this film