Two years ago I strongly considered using Perforce in the cloud via assembla. But two things stopped me, after some investigation:
1 - I couldn't easily migrate my existing depot into their service.
2 - I wouldn't be able to migrate my depot back to a local server, if I wanted to do that at some point in the future.
I'm super excited about the Helix Cloud - but I'm hoping it has planned around fixing one or both of these problems, so that I don't have to worry about whether or not I'm shooting myself in the foot.