I've heard some negatives about Bittrex because of their required authentication process before withdrawl. Also, it looks like they're accused of several price manipulation cases.
And there's a shit ton more if you do a quick google. This doesn't pertain to 95% of traders on Bittrex but it's just something to look out for. As far as if they'll remain in service, I'm sure they will. The company is making so much money from all the fees that the only thing stopping them would be a hack.