It's great.
The only disadvantage that it has over physical games is transferability - I can't lend a game to a friend very easily, as I could with a physical disc. (I'm not talking about both playing it simultaneously, I'd be happy to deactivate it from my PC at the time and have only one active installation - just as if you were lending a book to a friend).
Other than that, I have no complaints. Games are about half the price of retail (in Australia), and the sales are awesome. The potential for indie titles to reach a market is also great.