Path of Exile is hands-down my favorite of the Diablo-style Action RPGs I've played. To be fair, I don't have extensive experience with the genre, having played this game, Diablo II and Torchlight II (and Bastion if that counts). While I enjoyed Diablo II, I ended up quitting Torchlight II and Bastion before the end. There just wasn't enough to hold me. Path of Exile, however, has held me tight and I have no intention of quitting anytime soon.
The game plays a lot like Diablo II, with a few key differences. You walk around randomly-generated locations, killing swarms of monsters and collecting the loot they drop. You advance across the different levels and acts, leveling up your character as you do so. There is always a hub town in each act which serves as a place to heal, buy and sell equipment and collect quests. So the overall structure of the game is the same as in Blizzard's classic.
There are some changes which differentiate it from that game and end up making it better. For starters, your character doesn't gain spells or abilities through leveling up; rather, you find gems which you can slot into your equipment and which provide you with abilities to use in combat. I think this is brilliant because it allows basically any class to use any spell so long as you have the stats to support that gem. It makes your characters much more customizable, without having to neatly fit into any typical class style.
Your characters also distribute talents differently than in most games of this sort. Whereas most games have individual talent trees for different classes, every class here shares one (albeit massive) talent tree which distributes passive bonuses, with each character starting in a different point on that tree. So again, you can color outside the lines of your character class, so to speak, and you end up having more freedom to explore different playstyles.
An interesting aspect of this game is that the levels you go to are randomly generated every 15 or so minutes. Unlike in Diablo II where levels are randomly generated the first time you enter an area and stay that way for the duration of the game, the levels here are randomly generated pretty much every time you enter. This can be confusing at first, but it does end up making the game a bit more varied, so that even if you have to go through an area again you don't feel like you're just retracing your steps.
The gameplay is quite solid, very much akin to Diablo II. There really isn't anything to fault there, and if you enjoy that type of gameplay you'll definitely enjoy it here.
The story is pretty threadbare. You play as an exile in an island where all exiles in this world are sent to go. You end up traveling across its different realms for... what exactly? The game doesn't relaly do a good job at explaining the reason. It definitely emphasizes gameplay and atmosphere over story. If that's important to you in these types of games, Path of Exile might leave you wanting. Personally, I think the gameplay is solid enough that the lack of a story didn't bother me as much as it might have in other games.
The graphics are quite good, especially for a free-to-play game (they're better than Torchlight II's imo) and the art design is well-done too, a kind of dark fantasy aesthetic. Music is more subjective but I think it's quite enjoyable.
Perhaps the biggest reason to pick up this game, however, is that it's absolutely FREE. There is no way to buy power in this game using real-life money, as the only things you can purchase are cosmetics. I have not yet paid for anything in-game and I've never felt like I was being held back for doing so. The business model is exceedingly fair. So if you have even the slightest interest in this game, do yourself a favor and download it. You might end up loving it.