I'm torn
I loved Enterprise, in fact it's my favorite ST series.
But, I can see where some of the more devout followers of canon are...not entirely comfortable with how Enterprise fits (or doesn't fit) with the rest of Trek.
I've never felt it was really a big deal. Had the third season of Enterprise been about the beginnings of the Terran-Romulan war, instead of the Xindi, I wonder if attitudes might have been different. I know there are other issues, but if I had to point to one thing that doesn't seem to fit into the universe established by TOS/TNG/VOY/DS9, I'd say the Xindi arc (even though I thought s3 was awesome).