It's canon, and I've yet to see a plausible argument as to why it shouldn't be. And, no, the ship name and the fact T'Pol didn't have slanted eyebrows in the first two seasons do not count. My problem with the canon and not canon debate tends to come from the fact that people look for a reason why it's inconstant (T'Pol's eyebrows aren't slanted, she was in Starfleet, the ship is named Enterprise). I can admit, the show had issues, but it's not the first trek series to have done so. I'm sick and tired of Enterprise being treated like it's the series that killed trek, or that it was a total fuck up every step of the way since day one. The other shows have created plenty of issues (TNG's the Klingons joining the Federation, anyone?)
Honestly, I give the its not canon crowd very little credit, considering some of the things they bitch about. Again, as others said, TOS was inconsistant with itself (somewhere), TNG didn't mesh with TOS 100%, DS9 didn't mesh with TOS and TNG 100%, VOY didn't mesh with TOS, TNG, or DS9 100%, it happens. Sure, it sucks, but it happens. Also, a lot of the "its not canon!" people tend to cite facts that are not at all canon to begin with, so...
Honestly, I give the its not canon crowd very little credit, considering some of the things they bitch about. Again, as others said, TOS was inconsistant with itself (somewhere), TNG didn't mesh with TOS 100%, DS9 didn't mesh with TOS and TNG 100%, VOY didn't mesh with TOS, TNG, or DS9 100%, it happens. Sure, it sucks, but it happens. Also, a lot of the "its not canon!" people tend to cite facts that are not at all canon to begin with, so...