I'm going to ignore the bugs, the terrible UI, and the performance issues for the purposes of the review -- they can be patched out eventually, but if I was taking them into account I'd score this maybe a 3 if I was feeling generous. Developers need to stop releasing games that are broken, but that's an argument for another day.
The story is so-so, and the fixed first person perspective makes the storytelling feel awkward and flat -- most story moments are relegated to expositional dialogue, often with you and other NPCs awkwardly rooted to the spot, making the whole thing feel a bit like a Bethesda game; it was kind of lame and daft in Oblivion back in 2006, and it's not got any less lame since then.
Whether you're using ranged weapons or melee attacks, combat is wonky. Aiming guns often feels sluggish, melee combat is far too chaotic to approach with any finesse, and the enemies feel distinctively bullet-spongy. You can sometimes avoid combat with some hacking and stealth, but the stealth is pretty crude and the hacking is both overly simplistic (not many options available unless you invest very heavily in the right bits of the skill tree) and overly complex (the UI is fiddly, and targeting things to hack sometimes means you need to be out in the open ruining the whole stealth bit). Unfortunately there are some set pieces that require combat, though, so if you've spent all your points going for stealth you may find things get a little tricky. (To make it worse, skills are gated behind attribute levels and there's a hard level cap, so if you put a lot of points in to the tech/cool attributes you're boned -- there's no reskilling available.)
Now the open world. It can look impressive, and it appears to be busy and full of life, but it doesn't take long to discover it's all a facade -- there's very little you can actually interact with, the NPCs wandering around don't react convincingly to what's going on around them and seem to spawn in and out randomly, traffic literally only operates when you're looking at it so if you look both ways to cross the street cars will appear and disappear every time you turn your head, and it's all very immersion breaking.
I've replayed Witcher 3 at least dozen times, but this I had to force myself to finish and am unlikely to be picking it up again any time soon.