Gothic II is an excellent game. Period. If you missed it when it first came out and you are a fan of the genre, definitely check it out. I haven't played the original Gothic and I've only demoed Gothic 3, so I can't compare Gothic II to those, but it certainly stands on its own as being well worth the money.
I'm often put off by the outdated graphics of GOG titles, but the graphics in Gothic II actually hold up pretty well for a game that originally came out in 2002. Plus, the gameplay and story more than make up for any of graphical shortcomings. In fact, Gothic II does a lot of things better than many newer RPGs and, if you can get past the initial difficulty of the game, you'll probably find Gothic II very rewarding.