As has been said, it's not a new concept. A lot of these Free to Play games charge an early access, selling Founder Packs and whatnot. It only just went into Beta though (it was a 'Technical Alpha' before)
For my 2 cents I'm really enjoying it. As a palette cleanse brawler it succeeds. It has certainly got the highly polished Blizzard sheen to the edges. But it is certainly not $39.99 worth of entertainment yet. You unlock chracters at a pretty steady clip with the free currency, and the fact you might not actually like the unlocked characters, it's a bad value proposition.