I've only ever played Halo 1 (combat evolved and custom edition) on PC, and I loved them. The gameplay (single or multiplayer) isn't anything fancy, but that's just why I like it. The MP is a nice, simple FPS, with a good variety of weapons and vehicles too. The SP is a good game, with nice cutscenes, which has pretty good replayability, and an excellent background story to it all. And the Halo universe is a large part of why I like Halo so much - IMO Bungie has done a really excellent job in 'fleshing it out', and given the entire series way more depth to it.
In case any of you wonder (not sure how many people know about it), 'custom edition' is like an addon to Halo 1 PC, which lets modders put in all sorts of cool stuff - massive maps (like 8km across, sometimes), awesome vehicles and weapons (such as flyable Longswords, Pelicans, Seraphs etc etc) and just overall win. When I was last on a few months ago, Halo 1 had several hundred people on it, and custom edition had a bunch of full servers (and those were only for the maps which I had) - I think that says something good about a game which is soon to be 10 years old.