While we all have to admit CDP did an oustanding job at bringing Sapkowski's imagination into games. The atmosphere is spot on, the music absolutely fits, the characters immediately reconizable... there are still some differences between the main protagonists. Here I'll try to sum up a few key differences between Geralt in the game and Geralt as described by Mr. Sapkowski:
Book Geralt isn't as buff as Game Geralt, in fact, he's being described as rather "lanky", "slim", or full of sinews. Milva, upon seeing the witcher's upper body while he was resting in Brokilon, thought to herself that he looks like a proper man, albeit rather skinny, and full of sinew.
Book Geralt's signs are basic at the very best. The signs in the games are just beyond OP. No mage in the books has the capacity to mind control anyone, yet Game Geralt does it with ease with Axii. Aard signs are used to throw opponents off balance in the books, sure, but never multiple foes at once, and never with such magnitude as found in the games. Igni can be used to light up fire and burn ropes, but that's it (although we can argue that the power of Igni and Aard are for gameplay purposes only). I think the most realistic portrayal of Igni in combat is when Lambert used it to blind 3 Wild Hunt warriors during the Battle of Kaer Morhen.
Book Geralt can get beat and does get beaten fairly often. He was crippled and spared by Vilgefortz. During his fight with the mage Rience and his 4 lackeys (professionally hired assassins), Geralt was mortally wounded and would have bled to death if not for Shani and Philippa's help. In the newest book, Season of Storm, Geralt was beaten to bloody pulp by a female guard-captain (think Brienna of Tarth size) and her 4 other, also female, "co-workers" in an unarmed fight. Judging from the cutscenes and the mainstory found in TW 2, Geralt had no trouble slaying 2 witchers in a 1v2 fight. This is a feat I don't think book Geralt is capable of replicating.
Personality wise, book Geralt is rather sulky, gets offended easily and has somewhat a dosage of self-pity. He repeats the phrase "i'm just a simple, dumb witcher" a lot. And he does sulk from the his friends from time to time. Game Geralt, well, I'd say he's less dramatic in that department.
Anyways, those are all the differences I can find. Feel free to add yours. Personally I like both Geralts. Each has his own strengths and weaknesses, and I don't think we'll find any clear conclusion as to who is better.