Dang, looks like I missed a fun discussion here. Guess I'll just recap my thoughts on the mentioned titles:
X8 - Hated the art design, was a shame about armor laziness (I blame unwillingness to draw extra mugshots, which you can see carry over into MHX), Zero's dash stinks, and I wish the boss music didn't change mid-battle (the earlier version is so much better). But gameplay-wise it's pretty cool, makes excellent use of New Game Plus (buy all shop crap and leave the capsules; makes Replays a lot more fun), and I love that they gave the Ride Armor its own badass theme song and FRIKKIN INFINITE GROUND DASH!!! I've been waiting for that feature ever since watching Zero dash across the screen in X1.
X7 - Would have worked if Capcom had committed to either 2D or 3D. Their inability to do a proper merger shot the game in the foot (and IMHO they were closer to getting it right in 3D; the game's 2D level design is absolutely horrible, Stonekong excepted). X needs to stop whining, of course.
Z series - Great, although I find Xtreme2 superior. Gameplay-wise, it strikes me as X with better physics but watered-down features. No air-dash, no Earth Gaizer or Double Charge Wave IN ZERO'S OWN FREAKING SERIES, took them until Z3 to do the Double Jump and it barely gets any height, the weakness system is just recoloring your charge attack, the ranking system is senseless, and the art style blows harder than X8's. But the story, though jumbled at times, is awesome, as is character depiction overall. Zero has great dialogue moments in all four games. And even though EX Skills weren't always the greatest, I have to give props where they're due to Z3's V-Shot and Throw Blade, those attacks rocked all socks. Most importantly, the music in the series is in-freaking-credible. Straight Ahead remains my favorite stage theme of all time, and others such as Departure, Trail On Powdery Snow, and so entirely too many others to mention were instant classics. And, last but not least, Inti did in one attempt what Capcom failed to do twice: They actually did a good job of killing Zero.