I'm at the start of Act 3 with my Embermage, on Elite. It's not really "challenging" as such, since you can die as many times as you like. I've died 20 times in some sections or bosses, mostly because the location wasn't big enough for kiting. Hardcore would probably be actually challenging, but it seems it's really easy to level up on areas below your level, so I'd assume that's what Hardcore boils down to.
I've mostly spent my points on DPS. I didn't start spending on Vitality until the 2nd half of Act 1. I had the teleport passive melkathi mentions, but found it more annoying than useful (I actually also had a unique staff that did that on 30% of hits, very annoying). I used to invest alot on mana and mana regen stuff, but noticed that if you spent a lot of your points on Focus (even 3 per lvl should be enough) you have so much mana anyway that those becomes meaningless. Recently I mostly invest on damage bonuses.
I wouldn't be having half the fun I have with this game if I hadn't cheated myself a bunch of respec-potions, though.