I've been playing Heroes of Newerth, and I got to say I'm hooked. I love it way more than DotA, all the improvements and such in it are fabulous. Some of the heroes (Thunderbringer, The Madman, Defiler, and Moon Queen especially) seem utterly ridiculous at this point, but I'm told that's because Thunder + Madman's DotA counters aren't in yet, and Defiler doesn't have the changes from the latest DotA patch so yeah. Moon Queen is only broken imo because everything stacks, so you can get massive life steal & mana burn on her glaive attack and she just shreds through people like butter, combined with her ultimate and you have an insanely good carry. Some of the new heroes (Scout especially) seem bad/mediocre though. Pubs keep complaining about how OP he is, but they never take Wards or Dust. Ever. I think in comparison to DotA though, it's better off in balance at this point, mostly because there are less heroes and the ones there are were tweaked in some ways.
S2 Games is a really good company too, so I hope it gets a lot of support, they've said this is their last game and if it fails they got to call it quits. It's so good though, and in a very established genre so I have no doubt that if they advertisize it right, they're good to go.