I grabbed Gothic 2 last night, and am wondering how the first compares.
I'm a bit of a completionist, so I like to collect up whole series if I acquire one of the entries.

they use the same graphics engine. gothic 2 has a slightly improved combat/movement and inventory systems. The story is direct continuation of gothic 1, so if you play them in reverse you might know some things about the first game (probably nothing major) and won't have any clue what's going on in the second. You also revisit some of the areas from the first game, which is cooler when you already know the lay of the land.
In terms of story/plot, I think they're both pretty good. Gothic 2 may be a little better, but both games are definitely solid. If you play 2 and like it, you'll definitely like #1 unless you REALLY hate the control scheme (its not that bad, it just takes a bit to get used to it)

I almost gave up on it due to the controls, but the more I play, the more I get used to it.
Would it really have killed them to include a mouse cursor though, even if only for menus? :P

Both are very similar. Though Gothic 2 is bigger and improved in most aspects, Gothic 1 has a more foreboding atmosphere. You start the game with no friends and no belongings and just about everything can easily kill you. By the end of the game you are almost untouchable.
I prefer the first one just for the atmosphere and sense of accomplishment.
For the price, they are both a steal if you like RPGs.

Yeah, I went ahead and grabbed the first one as well, Fortunately I wasn't too far into the sequel, so I'll just abandon it for the time being, at least until I play through the first.

I like the more cruel start in 1, and the way you have to be everyones sucker just to get around. But 2 is still VASTLY superior to me (it's also my first Gothic game, at any rate).

I can only agree with other comments: if you enjoy Gothic II, you should enjoy the first opus as well.