Joel just sent me a Google Wave invite (thanks, Joel!) Been checking out google wave for about 45 min, seems cool so far. I am pretty nervous about instant messaging in general though, cause it's always so "in your face" when you've got thousands of contacts potentially all writing to you at the same time... it becomes overwhelming in those cases. That's why I literally never run Facebook chat, and why I am invisible in Gmail chat or invisible on Skype. Alternatively, I just turn those tools off when I am not logging in specifically to talk to someone.
This sortof brings up the issue of instant gratification in today's society ;) I like the letter writing style of email in general, by not having an instantly-viewable conversation, email lets you word sentences a certain way, give a certain focus or intention to your words, and really focus on why you're saying something and how you are trying to say it.
Instant Messaging is more akin to chatting with 1000 people, all at the same time, and each person pulling your in a different direction. I always get way too flustered with instant messaging, and that's the main reason I stopped using ICQ and Windows Messenger ages ago - it's just too much to handle all at once. When I go to write an email, I am aiming to communicate calmly with one person (occasionally more), and I want to write a letter at my own pace and send it off once I've proofread it (to make sure it doesn't sound stupid or imply things I didn't mean to imply)... so for the Google Wave folks to say that wave will "completely replace email" seems a little far-fetched (at least in it's current implementation). I think there will always be the need for quiet, private conversation, and it seems too nerve-wracking to have someone potentially peering in and reading my sensitive email while I am in the midst of writing it to them!
An area where I really DO see using Google Wave is collaborative stuff - brainstorming, working with co-workers, planning classes and curriculum... this would be huge. It could even be a more exciting/interesting way to deliver blogging to friends and family. This is sortof like sending a mass email but you get to drop photos and comments and stuff in the "wave", that is actually VERY cool. I also see this as the real implementation of google docs - this is the way that people will actually use Google Docs for business meetings etc (and Google could take down MS Word with this). Pretty cool stuff.
One area where Google might benefit is that I could definitely see using this for work, but definitely not with proprietary files or company information. I don't think this is a plausible tool for a lot of businesses yet, until there is a "private" version that doesn't store any information on Google's servers. But until then, I can definitely see using this as a personal communication tool (rather than sending multiple text messages, for instance). The "invite" feature is awesome, and totally wipes the floor with Facebook's Events feature.