I really, really like Ground Control, and got it pretty early in it's life.
I only recently got hold of GCII, and although I find it to be a nice solid RTS, I think it loses all the feel that made GC so good. Also you can now build units, which in my opinion just utterly ruins the whole concept of Ground Control is the first place.
So while I think GCII is pretty good, I much prefer the first one.
Although with the first one, I have this strange problem for the entire time I've had XP, where nine times out of ten the briefing voice files would skip till the next one has to play. The patches do nothing. It's really annoying, but that's the only problem I've ever had.