I think they mostly get Tyrion's wittiness spot on in the show, but he might have said in the show what was in the books verbatim, about one of the dwarves being mad with lust which landed him the role as the cupbearer, I don't think much was off.This is not a spoiler but just an observation about differences between the books.
If I remember correctly joffrey was an even bigger prick to tyrion at the wedding and tyrion was far more mouthy because he was completely wasted. I wish they would have put that in the show. Also, oberyn was not as mega gay as he is in the show. I find it super annoying that he is because it just seems like a play at being controversial and appealing to gay viewers. It comes off as over the top and offensive to gay viewers if you ask me. Maybe a gay person would disagree with me, but that's my opinion. He's like this stereotype of a gay rapist and it just doesn't gel with his character in the book. He's also older than I imagined from the book.
Gay rapist? I don't see that from the scene in the brothel, but yea, I don't think it was the best way to portray his sex drive.