Ill just put it like this: this is a true successor to the first two gothic parts. Thats really all you need to know ! On top of that, its pretty much bug-free as far as i can tell after 30 hours of gameplay. Also: if you like this, check out the mod "chronicles of myrtana" which is as awesome as this one (not available on gog though).
I have to admit, i was not 100% sure about that one since the witcher 1 and 2 - allthough great games - didnt really catch me. But because of the good reviews i had to give it a try. And it ist certaibly the best game i ever played. Maby together with Gothic 1 and 2 - my alltime favourite games. One quick tip: imo you should definitvely play at least on the second hardest difficulty because it actually adds a lot of contend and game value. Why? In the lower difficulties you allmost dont need sings and alchemy, to survive, but they are a big part of the charactersystem. And managing the second difficultiy isnt that hard. I was able to, and i never was the guy who liked dark souls - because i found it to be too hard to enjoy. I thought i would never find a gem like this again (since gothic 1 and 2) but this certainly is one!
