I'm absolutely in love with this game. I just made it to Skellige, after uncovering every question mark in Velen/Novigrade and doing every level-appropriate quest there. I'm playing on Death March, and so far I haven't really felt the need to turn down the difficulty. There's been a couple of harder fights, but nothing I couldn't eventually brute force my way through.
I started off really on Quen obsessively to stay alive (you die quickly on the hardest difficulty), but I've started throwing some other signs into the mix as well. Igni's alternative mode, a stream of fire, is an absolute pleasure to use against bandits. Axii is also nice against larger enemies, as a stun.
Honestly though, I spend most of my time obsessively playing gwent, forcing merchants and innkeepers to give me their cards, as I try to amass my collection.