Spent a good 4 hours playing it last night, exploring around the starting area, taking things slowly, doing some exploring, combat, questing, and spent some time fiddling with the crafting and other parts of the UI. So far, I'm absolutely loving it, the atmosphere, characters, storyline, and immersion are fantastic.
The combat difficulty seems fine on blood and bones, quite challenging, however there are a few niggles.
The lock on system is a bit pants, losing your target or not selecting the correct one initially is annoying, and I'm finding it tough to time my counters with the parry ability (can you parry monster/beast attacks?).
Performance wise, my R9 290 handles it brilliantly. I've not measured the FPS I'm getting, but I'm running @ 1080p with everything on (inc. post process stuff) & on ultra except shadows and foliage on high, and hairworks off, and the game runs perfectly at the moment. The sheer quality and scale of the game as a whole makes the graphical niggles pale into insignificance (that low-res grass aside), which makes them easy to quickly forget about (or not even notice at all); my console playing house-mate was absolutely blown away by the visuals..
Massive admiration and appreciation to CDPR for this stunning masterpiece, I can see it being an incredible game, with non of the generic hunt & collect quest fatigue I usually get from large, open world games.
There's some good guides out there already on the web worth checking out, stuff like being able to drop all the books / letters you get in a spot on the floor to free up inventory and keeping the witcher armor as you can craft high-tier armor with it later on for example.