If anyone has, can I get a comparison? Especially from those that feel D3 is lacking what they had hoped it would be?
Path of Exile wins in terms of customization and character choice, to me. D3 doesn't even begin to let you have control like that. Path of Exile will have you thinkin' about what you want to do every level or so. Diablo 3, you'll take 20 minutes a few hours into the game to consider how to put together abilities, runes and passives, but very infrequently. By Level 25, you've read all your abilities, all your runes and all your passives and know what they're going to do, so you don't even need to take a break when you level up, unless you're plannimg on using those new unlocks or have gear you've been waiting to equip.
D3 wins in atmosphere, no surprise there. Path of Exile has its own look and feel and it's not bad, it feels very D2 old school.
Path of Exile can seem bland compared to D3, because it's doing what D2 did pretty much to a T. On the other hand, when you've played the same set piece 4 times with 2 character in D2 (and a set piece generally has a minute or less worth of stuff going on in it), it can seem bland as well.
PoE barely has a story but it's incomplete and very vague, compared to D3. That's only a minus if you think D3's story was good and cutscenes actually matter. Otherwise I liked that PoE wasn't beating me over the head with its story.
I've put far more hours into D3 than PoE now, so I can't claim I know them both very well. But bottom line, one is totally free and requires always-on for legitimate reasons....and one is $60 and requires always-on for several BS reasons. The only reason I stopped playing PoE was because D3 came out.
I also prefer PoE's item model. I'd rather use orbs of transmute to re-forge/improve items than the slog D3 has set up. Where you have to collect gear, to disenchant gear, to make gear which you may not want in the first place. And then you add Blacksmith pages on top of that to level up your crafters.....and it feels way too much like a WoW grind. What Blizzard is doing is fairly obvious and gear is both the carrot and the stick to them. PoE isn't gaming their customers quite that hard.