Loved the X series for how it went into a darker realm of dealing with true AI robots overtaking the world.
It was an interesting gimmick at first, the whole "hardcoe Mega Man" thing, but IMO, you can only do so much with that. Especially with the voice acting Capcom put into the later games, yuck.
I lost interest after X4, same as you.
The Classic series retains my interest though, simply because the story is minimal so the "re-hashing" of said story isn't a big deal. I'm liking how Protoman has been given main protagonist now. Hopefully in MM11, if it ever comes, the trend continues. Also, I wouldn't mind seeing Bass come back as well. 3 difficulty settings and 3 characters makes for a nice large variety of play throughs along with the new focus on "achievement runs" which I actually find enjoyable. Gunning for and completing the Mr. Perfect achievement really lets you get a deeper appreciation of the games.