Definitely wasn't make or break, but I've certainly been transitioning most of my assets out of Polo because of this and some other oddness I've seen there for the last month+.
They can't be beat for liquidity, but I can't imagine they're not complicit to some extent in market manipulation.
May I ask what exchange you are transitioning to?
I have used Poloniex for the past couple of months. Only had a few issues until this week where it now seems like the lagging is affecting trade in many altcoins. Have a little on Bittrex which seems to be working good. I don't want to get into exchange hopping but I probably should spread across more exchanges.