I was lucky enough to receive a Google Wave invitation in the second round of invites google sent out.
After the initial excitement of being one of the lucky invitees wore off I lent a more critical eye to what Google’s touted as an Email replacement.
So far to me it’s looking a lot like a combination of Instant Messenger, MySpace, IRC and WhiteBoard. I’m not seeing the Email replacement so far for one reason and one reason only. There’s no way to add someone to a wave without adding them to your contact list (At least that I’ve found so far in my incessant searching).
To me this is a deal breaker for an Email replacement, especially if you send a lot of messages to people once and then never need to contact them again. I can see the contact list getting WAY out of control. Hopefully this will be changed in the final release.
Other than that I can see Wave being incredibly useful for anything that requires a lot of back and forth or collaboration.