Through Normal difficulty, Path of Exile is an amazing game. The character build paths are numerous and distinctive, the item/gem mechanics are interesting, and the skill tree is a genuine work of art. It beats D3 soundly.
But after the end of Normal, PoE starts to seem a little lackluster. I finished Normal at level 35, and I won't see any random maps until the end of Merciless at about level 65. So that's 30 levels -- and two full playthroughs -- to go with no new content. Add to that the fact that character builds that worked at level 35 will probably fail at high level, and specific unique items may be required for high-level builds. I just don't feel compelled to stay with the game. The high-level PoE game looks very intimidating and not very fun.
That's not to say D3 is any better; it's still grinding the same content with very little fresh loot. And D3 (plus the expansion) is expensive while PoE is free. I just wish there was more to do at high levels in both games. Adventure mode sounds like a step in the right direction.
For casual players who don't intend to play through either game the expected three times, I heartily recommend Path of Exile. The new D3 content doesn't make up for the flaws in D3 -- the characters, skills, and combat mechanics are still poorly designed and lack the appeal of D2. PoE has that appeal, plus some innovative charm of its own.
For the heavily invested type of player who loves the grind...I don't know which to recommend. I'm not that type of player.