I love it! It feels exactly like a game from back in the heydays of the genre but is at the same time very playable (imho much more so than the old Wiz 7 which seems to be Grimoire's main influence). Be warned though: The game is hard as nails and will need some time to get into but after that it opens a world full of wonders, intricate puzzles and great possibilities. It's a modern classic in my opinion.
edit, after almost three months since my "first contact" with Grimoire: This game is massive! I'm still only around 20 % of the whole game (says the counter) and there is no end in sight. What's the most impressive for me: It keeps me hooked: My party really grew on me, I still enjoy exploring the many areas, collecting treasure, discovering secrets, finding the forbidden magic. And I enjoy the writing, it's well written prose and a pleasure to read. I immediately liked the game when I first tried it three months ago but now I'm sure I'll keep it with me for the years to come.
edit2 - after six months in Hyperborea: I'm around 56 percent now and still fully engaged. The game maintains its high quality in writing and dungeon design. After hesitating at first I now really enjoy the possibility of multiclassing and was able to create all the prestige classes with my party (the Jester is a real killer). I don't remember a single game where I invested so much time and felt so perfectly entertained for so long.