On one hand:

a) Excellent representation of the comic-book characters, both in terms of image and personality. It's a huge attraction to me because I used to read the comics, and I think fellow readers (or ex-readers) would also enjoy this aspect.

b) Lots of innovative touches, such as dressing Player 2 characters in the movie's costumes, even if they're much inferior to the comic-book costumes, and offering up great cover art for X-Men past issues during the (pretty long) loading times.

c) Including the movie trailer, even if I think the characters' image created for the X-Men movie is laughable, Cyclops, Magneto and Jean Grey being especially dorky-looking.

d) The inclusion of Phoenix/Jean Grey, one of X-Men comics' most complex characters (and my personal favourite), but rarely used in X-Men-related games.

e) Beautiful graphics overall.

f) Innovative "multiple-level" super-move system.

g) Great backgrounds.

Countering these strengths:

i) Sluggish response. It's hard to take advantage of an opponent's mistakes because of this. Also, the "hard" attacks become nearly useless because they take forever to come out.

ii) Imbalance. Mystique's gun is overwhelmingly powerful, and Beast can pull off a barrage of nearly-zero-recovery attacks with little risk to himself, but Storm and Gambit are too weak.

iii) Too little push-away for ordinary attacks. The chain-combo system leaves plenty to be desired.

iv) Some computer characters are pretty cheap -- Toad will toss you three times in a row if you don't retaliate, and that is made difficult by the slow response.

v) Terrible, terrible manual.