Gothic and Gothic 2 are two of my favorite RPGs of all time, and Gothic 2 is one of my favorite games of all time. I must have played them four times each, which is saying something for me because I bore easily. I truly loved these games. And I'm not one of these people that every game they play it's like “OMG, that was the best game ever!!” believe me.
If you haven't played any of them but like RPGs you'll find them quite different from most RPGs you've played. They're a little rough around the edges in terms of voice work and some of the interface screens but they're minor complaints. The combat system takes some getting used to, but if you stick with it and become good at it, it becomes tremendously rewarding. You'll be absolutely thrilled when you finally take down your first orc!
Have a look at a demo and give it a try. They're quite challenging games, they make you work. But if you stick with it and don't mind the slightly dated graphics you're in for a real treat. I wish I'd never played them before so I could have the pleasure of playing them all for the first time one after the other.
On a personal note I think G2 is better WITHOUT the expansion installed.