Whether I have a "personal canon" or not, doesn't really matter. It would be like whether I decided the Civil War never happened. Whether I acknowledge it or not makes no difference to it's existence.
Well, some people think that the moon landing never happened. It matters to them.

But you're right of course, there's no such thing as "personal canon". It's interesting though for what reasons someone refuses to accept "Enterprise" as Canon. Some do it because of continuity errors (like that didn't happen before...), others because they think "Enterprise" was simply a bad show (like this thread's OP).
This raises a question. If a show is full of continuity errors and retcons but has top-notch stories and actors is it more likely to be accepted as "Canon" than a show who fully and unconditionally adheres to previous continuity, but has mediocre plots and actors (or the other way around)? Maybe we should do a poll about it.
As for "Enterprise"... I refuse to acknowledge the existence of "personal canon" myself, because the term constitutes an oxymoron. That being said, the first two seasons of the show were mostly bland and uninspired. Simply a waste of time. It definitely got better from Season 3 onwards. But I guess it was too little, too late at this stage.
However, it wasn't necessarily a mistake to do a show right after "Voyager". The mistake was to have it produced by Berman and Braga (or any other people previously involved with producing Trek so much). Right from the start, they should have done what they did with ST09: Bring new people in. Get fresh prespectives. If anything, Manny Coto should have been showrunner from day one. Just like the Reeves-Stevens' should have been on board from the start as well. Paramount's mistake was that they neglected to do a real shake-up.