Despite all the current hype, the client is pretty damn useless, since it will only talk to Gizmo's own service. So much for their touting Open Source and standards compliance - it's impossible to establish whether or not it's truly standard unless it can connect to standard SIP servers.
Update: Beware their Spam mail. Despite not wanting to receive any updates, I still got stupid messages listing "ten tips to do with Gizmo" and other junk. For me, that's more than reason enough to avoid their service.