We switched from HipChat recently, so far it's working fine. The only thing we miss are cross team notifications, but apparently they are working on it (can't find the link at the moment).
It's possible, see https://slack.zendesk.com/hc/en-us/articles/201727913-Connec... for more details. You'll lose the history though. We've been using native clients more or less, all are working fine.
tl;dr: it's awesome
I've used it, and it seems pretty good, but I don't have enough time behind the wheel to provide a review.