I never liked Mega Man X3, I felt it was cheap and poorly put together, with "challenge" coming from cheap enemy placement and cheap attacks. X and X2 had a more well-balanced challenge.
X really refined the Mega Man gameplay, with wall-climbing (of a sort) and dashing, in addition to a sleek yet still cartoon-like look that felt to me like an evolution in the game's art style as opposed to a drastic change. X2 refined it a bit more, though I actually prefer the original (my younger brother prefers X2). X3, however, we both agree is terrible.
I don't know what's been of the series since, but I am interested in finding out. I am not happy with the super-dark mood of Mega Man Zero, or its cheap challenge. I wish Capcom would make more Mega Man (spinoff or otherwise) games that played like X and X2, with the right balance of challenge, fun, creative level design, and an overall well put-together package.