It's the type of game you either never uninstall, or reinstall countless times, because it's just that damn good. I recommend playing this with the GMDX mod (you can find it on ModDB) for a more modern and HD experience.
Witcher 1: Plays kind of like a singleplayer solo MMO, it was great. Witcher 2: Secretly a tech demo for TW3, it was good, but I didn't care for it that much, can't say why (Hint: It was short and had stupid QTEs) Witcher 3: Your wallet bursts out of your pant pocket, flaps over to Geralt, and feverishly asks him "Why can't more AAA games be this damn good?!", and then promptly spits out loads of dosh gleefully at Geralt- I mean CD Projekt Red, and then promptly returns to your pant pocket, completely empty. You will lose *minimum* 100 hours playing this wondrously fantastic game, and it's damn good expansions.
I first found out about Divine Divinity a couple years back and was like "Hey, that looks kind of like Diablo 2, I'll check it out later." When I did get around to playing the first game, I fell in love with it, the first game was basically a more complex Diablo II, in terms of some pen and paper freedom, and environments (and even some visuals), although it had less classes to work with (Only 3), though the cool thing was you could choose which gender your class was, which also changed some skills you started with. The game started out at a fair difficulty, then got brutally difficult as time went on, but as with all good RPGs, it got easier the more you understood the game, and the more you leveled up and got better gear. The game had one *huge* act, and then a second smaller point-of-no-return act, with tons and tons of sidequests, as opposed to D2's less huge 4-5 acts, and relatively small amount of sidequests. DD's world wasn't randomly generated like D2's, but what was there was huge, and interesting to explore. Well I could blab for another several paragraphs on how great DD was, but now on to this game. Original Sin is a prequel to the first game and changes gameplay from ARPG, to Turn Based RPG, and damn, it worked pretty well. Battles are relatively easy at first, but as you progress, it gets pretty hard if you don't know what you're doing, as you need to really exploit enemy weakness, as well as using environmental hazards to your advantage. This game, like the first one, also has tons of sidequests, as well as really well done Co-op, I had a blast playing with my friend on a second playthrough, at least until he got bored and quit, oh well, his lost. Original Sin's main quest is somewhat similar to the first game's (Find all the dudes to perform a ritual to ascend to Divinity) to this game's (Find all the macguffins to restore your memory, etc, etc.) Overall, Original Sin is a really good comeback from a once dormant ARPG series.
