I played the Windows 98 port of the first X-Com game (with xcomutil installed, natch), but I also played the first and second Firaxis XCOMs (only finished the second, sans DLC, but have played quite a bit of it with War of the Chosen).
Myself, I think Firaxis did a pretty good job with them and with Chimera, which introduced some interesting mechanics, although I hope they come up with another mainline XCOM game.