I got an alpha key and played for about an hour.
For an alpha, this game is already exceptionally polished. Visuals are excellent and might be the best I've seen in a Diablo-like game ever, music is very nice in that it keeps the creepy, sombre mood of Diablo without just ripping it off, the setting seems interesting and rife with potential, and the overall feel and challenge of combat are just about spot-on. Even early on I have come close to dying, and things are not getting any easier. Class selection so far is mostly stock-standard, as is the character progression if you played Titan Quest, but if it's not broken...
The environments themselves lack random elements, but they make up for it in design. There isn't a lot of repetition and it seems like you'll be seeing new things every few minutes. If there is any copy-paste it's hidden quite well so that you probably won't notice it. Instead of endless expanses of wilderness like you got in Titan Quest, there is much more variety in Grim Dawn. Loot balance seems slightly better, too, in that I'm not finding magic items every 5 seconds. Despite the game using health regeneration, this only really works out of combat - in combat, potions can be quite limited so you have to be smart about when you use them, but you aren't forced to constantly go to town after every fight or two.
I have had a couple of issues so far. The first is a widely-reported and persistent video stuttering/freezing that seems to occur every few minutes and lasts several seconds - even worse is that the game continues running while the freeze occurs, so you can often find yourself surrounded by enemies if you freeze up in combat. The second is that the visuals, while fantastic, are occasionally distracting. Due to how dark and grungy the graphics can be at times, sometimes the enemies aren't distinct from the backgrounds, and containers just get completely lost. Even if you are constantly hovering the mouse around, picking out a pile of bones you can loot from one you can't is not much fun.
Also, sound effects are pretty weak and voice acting is extremely sparse. I assume this will improve over time but even a few more random voice samples when talking to NPCs and removing some of the obvious stock sound effects would be nice. The music is good but the rest of the audio can't match it right now.
Comparing Grim Dawn to Path of Exile, I'd say Path of Exile is slower and has a weightier feel to combat that's closer to the original Diablo, while Grim Dawn feels closer to Diablo II as far as character movement speed goes. It's not quite as fast as Torchlight II, but close. Diablo III is the slowest of them all I'd say, but it also probably has the best-playing combat, at least as far as skills and enemy variety go - Grim Dawn is a close second to it though.