Yes, back in the day, I was something of a comic-book dork. I'll own up to that.
So naturally, when a movie like X-Men 3: The Last Stand comes out, I generally line up with the FanBoi's and see it on the opening weekend. So I did. My expectations were actually pretty high - the end tease of X2 left me so fired up, as it seemed like a nod to the true fans. However, I don't really think X3 quite measures up - at the very least, I'm not satisfied.
As far as big-budget action movies go, it was certainly passable - there's all kinds of coolness possible within the X-Men universe, and they exploit a good deal of it. HOWEVER, the problem lies in that very richness of the back-story. I may not have intellectually understood it at the time, but at their best, the comics of my youth were well-illustrated morality tales. To a degree, this carries over into films, as the best adaptations (the first two Batman films, as well as Batman begins; whichever Superman involved him giving up his powers; Spiderman II as well as the X-Men franchise) tend to grapple, to a degree, with the "great power = great responsibility" theme.
And one of more involving storylines specifically implicating this theme was the whole Phoenix saga. While X3 does a good job of making Jean Grey's Phoenix incarnation suitably badass and unstable, it ignores the far more interesting and explicitly Faustian aspects of the original. I suppose it is unfair of me to evaluate the film in this light, but those stories (at least as I remember them) were so rich and textured that it's a shame to lose that.
Of course, as long as they keep making 'em, I'll keep seeing them.