Now that I've given it some time, here's my brief thoughts about the X subseries:
Despite the ups and downs and the few slumps, I had a lot of fun playing through these games. The gameplay was solid overall and the main characters feel great to play, the levels and bosses tend to be pretty good with a few exceptions, and I liked that the story did try to give a narrative that was fun to go through even if it can be pretty cheesy at times. I also liked how each of the ideas one game brought forth tended to be brought back and improved on in great strides by the end of the series. It was interesting to see how things like the armor advanced from being simple concepts to more fleshed out gameplay mechanics, and it made going through each game to see what changed for the better even more enjoyable.
It was not always perfect though, despite the gameplay generally being solid, some of the level designs and bosses really let it down at times and sometimes it could feel like a slog to play through certain levels. Additionally, while I liked most of the ideas that grew with the series, some I didn't really care for due to how they were handled such as the rescue mechanic. Usually they weren't enough to bring the game down fully, but in some cases it did color my opinion of them and left them feeling lacking (particularly with the weaker entries since I felt that their ideas were good, but had poor execution).
All in all, it's a fun subseries and I'm glad I picked up the full collection. It's a series that I've always wanted to play, but never got the chance to, and now that I've had that chance it's given me access to a set of games that are home to some of my favorites from their respective systems. Nothing else to say but here's my personal tier list for each game:
Loved it: X1, X4, X5
Liked it: X2, X3, X8
Mixed: X6
Burn: X7