I used the Cobalt RaQ4/5 migration tool that is in Plesk 7.5.2 (i guess its standard, or maybe its only in there if you have the applications pak). And it seemed to work fine.
The only problems i ran into was with the way that plesk see's users and domains, as opposed to the way Cobalt sets them up. But over all, the process with clean, simple, and easy.
My customers didnt even know things had changed!
The only real problem I had was one of my customers had a password of around 16 characters - and plesk didnt like that... it imported, but truncated the password.