X8 wasn't too bad, IMO. One key thing that bothered me was the stage design. 2 auto-scrolling and 2 ride chaser stages is pretty much overkill in that department. Also, I don't mind the switching mechanic, but I like having the L2 Button free for dashing, which messes that up for me. Seeing as all of the buttons are actually used in this game, reconfiguring the controls to my liking is a bit more of a hassle than I would like it to be.
I didn't find the music to be bad, but I think they focused a little too much on the rock aspect. Primrose and Dynasty were the only songs that felt different than the others, and you can barely hear the former with all the sound effects blasting from the game. I actually don't mind the fact that the boss music changes, at least not on Normal mode, anyway. On Hard mode, having to listen to that repetitive beat with 50 more health to go is pretty annoying.
The arsenal of a MMX game is often a deciding factor for the games enjoyment IMO, and X8's is pretty decent. There are some fun ones (D. Diamond, Ice Gatling, Thunder Dancer), and some stupid ones (Shadow Runner is a bit weird at times), but they tend to get the job done. This is also the first X game (possibly barring X6) that actually gives you a benefit for not having a special weapon equipped; the ability to charge and shoot at the same time turns X into a very broken character. The use of different weapons for Zero made the game more challenging (less is you used Sigma Blade); try knocking out all 8 bosses w/only the K. Knuckle, it's not exactly a cakewalk.
The armors weren't bad; the mix and match gimmick worked out rather nicely, although I prefer my armor whole. The designs wasn't too great, and making the Ultimate Armor another recolor was a letdown. I feel that the Hermes Armor and X-Drive are severely underrated. X-Drive is invincible on activation, allows X to take 75% of boss attacks w/o being hurt, and ups your speed. Forget the fact that his buster does weak damage on bosses, just rapid-fire them! The game as a whole was decent, but there was some room for improvement.