I've seen people do this right. I've seen *Blizzard* do this right. Diablo 3 does not do it right. It's less stable network-wise than almost any MMO I've played. It's not just that being able to start up your game is a coin toss where both sides are 'no', it'll kick you off in the middle of a game too because the connection is interrupted.
And of course because of its lack of a real save feature, getting kicked off means restarting from maybe a half hour's worth of play in the past when you hit the last checkpoint. Assuming you can log back on. Which you probably can't.