Pros:
Stunningly detailed world, massive amount of content
Great writing, interesting characters and complex decisions
Technically very impressive - Looks great, sounds great
Immersive, hard to stop playing
Definitely worth your money

Cons:
Numerous glitches, while none too serious, may break your immersion a bit
Horrendous combat system
Awkward character movement
Some mechanics are poorly explained/implemented

Overall:
I had a hard time getting into The Witcher 3 at first, and stopped playing it for about a month early on. This is because the early quests aren't particularly interesting, and the combat really annoyed me. However, I decided to give it a chance and, wow, I'm glad I did. There's so many things the game does well that it's hard to dwell on the negatives (even if some of the negatives are pretty annoying).

Loved the game's universe and characters, and for once felt like I was making real complex decisions, rather than just choosing between "Give puppy belly rubs" or "Punch puppy in face" (ahem, Mass Effect sometimes). Once the game started picking up (around the Baron series of quests) I couldn't put it down. I managed to forgive the atrocious combat system, and ended up having a really positive experience with the game. If you like games such as Skyrim, you'll love this game.

However, it's not perfect. The combat system really gets on my nerves. Fighting with Geralt sometimes feels like trying to get a fridge to do somersaults. There's so much "weight" to all the movement - so much sliding around and difficulty changing directions that combat is a real chore. I suspect the combat makes a lot more sense on a gamepad. Furthermore, the game has so many damn mechanics, and explains only about 3 of them. I didn't use a single potion outside of swallow, thunderbolt in the game unless I was forced to by a quest. That's partially my play style, but none of the potions, bombs, oils etc. really seemed to be worth the effort of gathering their ingredients, fidling with the alchemy tab, etc.

Even the basic way the character moves can be frusturating. There's one particular quest where you have to climb a mountain and get a scarf, so you can prove yourself to the Skellige islanders for the billionth time. I spent most of this quest hopping around in front of ledges rather than climbing them, missing jumps, sliding up and down walls, and latching on to every ledge except the one I wanted to use. I don't think this is a skill thing, as all the contextual space button presses dont' seem to work very well. It doesn't help that Geralt moves like you're controlling him via a faulty RC remote.

But overall, the game's positives hugely outweigh its negatives. My suggestion: Don't play this game right after playing Dark Souls 3, as you'll be spoiled by DS's finely tuned combat. If you go into it willing to put up with a little clunkiness, you'll undoubtedly enjoy the experience.