I like PoE so far. I'm like, level 16 and haven't finished act 1. I'm not even sure there are acts...
As far as game mechanics go, it's better than D3 in almost every way. I think that the inventory is a bit small. You constantly have to go back to town.
It's more fun to kill monsters in D3 though. It feels great and is responsive. Often, in PoE, you're not even sure if you killed the enemy or not, so you keep spamming spells... It's functional, but it could be much better.
My main positive is the fact that there's actual character progression and customization.
The always online thing is indeed a bit annoying. Especially since their servers constantly die and restart (beta). I keep losing town portals (not cheap in this game) and have to redo areas that respawned when I rejoined.