I personally liked it, and while I think it both helps and hurts to have a Gothic background before you play Risen in the end it ends up being a positive.
If you loved Gothic you'll probably find that Risen is not as deep as those games. For example: I was pretty disappointed to find that the swamp camp was just a short jog from the beach where the game starts. At first I felt like I was in a RPG amusement park instead of being trapped in a prison colony (I think it took me a week to even find the mercenary camp in Gothic 1). A person who hasn't played the Gothic games wouldn't have that issue.
But how much depth is too much depth? I've started Gothic 3 many times and enjoyed it (especially with the later updates) but I've never come close to finishing the game. It's just so fricking huge my games always start to lose focus after a certain point. Risen definitely doesn't have that problem. It's much more focused and restricted.
I was pretty shocked to read some of the stupid internet comments say things like "the graphics are worse than Gothic 1". I thought the graphics were good. The game did stutter a bit in the city with my 8800 Ultra.
As far as combat goes: I didn't do magic on my playthrough, I concentrated on swordfighting. Expect lots of tense, slow battles with both you and your opponent defending and trying to counterattack. I never did learn to properly time the move where you stun somebody during their attack, so I had to compete the game without it. All in all the fights were pretty fun.
If you like RPGs and aren't turned off by not being able to customize your character it's definitely worth it, probably my favorite game of 2009 (I didn't play many great games last year).
As a comparison: I liked the Witcher, but I liked Risen a little more.