Enterprise, as a stand-alone series was actually pretty good. Enterprise as a Star Trek series, though, wasn't. Unfortunately, Enterprise shot a lot of canon all to hell during it's run. For example, the first episode introduced humans to Klingons, which was supposed to happen long after the Romulan/Earth War. Another example, the Romulan/Earth war was supposed to be fought with nuclear weapons, and was supposed to take place prior to the Federation forming. Things like that. There were a lot of things that were established in other series (especially the Original Series) that was ignored by Enterprise. For that reason it stands out in Trek fiction as a huge canonical mishap.
That said, the last season of Enterprise really did a lot to bring the series back into line with established canon. In fact, I'd be willing to say I liked the last season (save the last couple episodes) more than the rest of the series. Not only did we take a look at the precursor to Journey to Babel (an excellent TOS episode), but we also peeked into the Mirror Universe and explained what happened in the Tholian Web. The explanation of the Klingons was also, in my opinion, top notch. It definitely could've been ignored (Worf's response of "We don't like to talk about it" suited me fine) but to do it, and do it well, that made up for a lot of the early canonical mishaps.
All in all it's a fun series with likeable characters. Unfortunately it lost a lot of Star Trek "cred" when it played fast and loose with the established canon (a lot like why any die-hard Star Trek fan will tell you that the newest movie either: 1. Sucked. Or 2. Was a cool movie, but wasn't Star Trek). Star Trek isn't meant to be completely accessible, in all honesty, and I think Enterprise did away with a lot of the good science fiction of the old series in favor of easily recognizable techs. Warp theory is a real theory, and anti-matter reactions are a real thing. Enterprise, more or less, neutered Warp theory and ignored making the ship's power systems and weapons match the time, and just grabbed technologies from other series. Canonically Enterprise should've been using nuclear weapons and the personnel should've been firing lasers, not firing "Phase Canons" and "Phase Pistols."
Finally, and this irked me most, the premise for the show was flawed. One of the main reasons that Earth is chosen to, more or less, lead the Federation was because Zephram Cochran built the "better warp engine." That was how it was explained in TOS. Instead we have Vulcans, Andorians, Tellarites, ect all with better ships than our measly one.
On a completely different note, I do have to admit that modern Trek has ignored established canon before. First Contact used Zephram Cochran's launch as it's central plot, ignoring the fact that Zephram Cochran was supposed to have been born on Alpha Centuri...but I digress.
I liked Enterprise, and I'm a die hard Trek fan (Trekkie? Trekker? Who cares?), but I would've liked it more had it stuck to canon a little more closely.