Clearly, the success of Spider-Man is written all over X2 - the increased budget, the far more expansive plot, the greater investment in character arcs and so on.
That said, X2 does walk a bit of a wobbly tight-rope between deeply-plot-driven, and simply-too-talky. The action is upped, but so is the running time by a considerable degree - and for the first half anyway, there's not a lot of bang to balance out the character exposition.
Then, particularly in the somewhat overlong third act, the pendulum swings in the completely opposite direction and large scale action mostly takes centre stage.
I've heard mixed reactions to X2 and X3 (I'm still yet to see X3) - some love X2 but hate X3, and some are of completely the opposite view. X2-wise, I did think it was perhaps a bit uneven - some more action earlier on, and less later on would have balanced the whole movie out ... and just thinking about it, the character arcs are a little bit mixed. Understandably, with so many characters, you're going to run into a problem of fleshing people out - I've experienced the issue myself when I was writing a script which had a whole list of characters - but what you do find is that some characters are underdeveloped, and even the ones who take centre stage find themselves cut adrift somewhat.
Mind you, I think X2 works - overall - better than X1 ... not that X1 was bad, not by any stretch of the imagination. X2 certainly feels far more epic, and the budget level feels far more suited to the plot this time around, when compared to X1 and I think that's where the difference lies.
Also, it's quite clear that X2 is the "second act" of a deliberate three-movie run ... and no doubt that's why there has been a varied response from fans. The second act is always the largest, and the one where the most exposition occurs. X1 - the first act in the trilogy, essentially - sets things up efficiently (although you do feel it would have benefitted from a larger budget), and X3 (which I am, at the time of writing, still yet to see) will be - I'd assume - the fairly efficient wham-bam third act loose-ends tie-up of the whole lot.
While X2 might be a bit flabby, it takes what X2 did and takes the simple notion of "more is more" and runs with it quite successfully - more plot, more characterisation, more action. Next up, let's see what X3 is like ... although it not being Singer branded X-Men, and having Vinnie Jones in a daft-looking costume, doesn't do it many favours from a pre-viewing stand point.
That said, I'll no doubt enjoy it - particularly as a three-movie closer - but, like Spider-Man 3, it will no doubt be "not as good as the first two".