At the risk of wandering too far off topic, I wanted to make a quick followup post to my last comments about the level design in Unreal games. The following sums up quite nicely what made Unreal levels so special and memorable:
http://www.unrealsp.org/community/leveldesign/articles/conceptualgrandness.html
From this perspective, Unreal II may have been the lesser game, since I can't remember any of the levels as well as from Unreal I, at least not by name. I do have pretty strong general impressions of Unreal II levels, though, so maybe my poor memory has more to do with the games focus on overall storyline rather than on individual levels.

So basically, play both? Haha.
Thanks for all the comments guys. Im basically new to the series but now I feel pretty educated and I think I'll be picking at least one of them up..probably 2k4. :)

Keep in mind that the "Unreal series" really embodies 2 very different kinds of games.
If you like single-player, story-driven games, then by all means play Unreal (Gold) and Unreal II.
If you like multi-player, online, combat-driven games with no real plot to speak of, then the Unreal Tournament games are what you want.
The Unreal Tournament games do have a single-player mode where you play against bots instead of other human beings online, but the game style is still essentially the same.
The Unreal "single player" games do have a multi-player mode which also supports online play. I suspect that the number of online players is much higher in the Tournament games though. My personal habit is to turn to Unreal Tournament when I want to play online for some mindless mayhem and Unreal (not-Tournament) when I want to play a game by myself.

I know. Thanks though triplett. :)
Not sure if Unreal is what I'm looking for in a single player shooter..and Im not sure if playing shooters online is my thing (partly cause I seem to be awful in comparison to most others). Sigh.

I must confess that when I have time for some quick gameplay (ie. not enough to get deep into a story) I just play UT (or UT2K) against the bots. Seems to take me about 20-30 mins to get in the zone online so I don't get my a$$ handed to me, so 30 mins against the bots is usually more appealing. :-)

I've still never played any of the Unreal games against human opponents (I never play shooters online, I'm just not good enough). And yet UT and UT2k4 are two of my favourite games ever. The bots are excellent.