It's been about a week since I first started playing with the preview of Google Wave. The first problem you encounter when you receive your coveted email invitation is finding anyone else who's got an account that you want to communicate with. In my case, I found that were quite a few of my colleagues with accounts and we quickly hooked up in an experimental wave.
The interface is reasonable, if a little complicated, until you get used to it, and the way it incorporates real-time chat into "email" is really impressive at first - until you realise that everyone in the wave can see your appalling typing and corrections as they happens. That can be painful.
The biggest problem, though, is maintaining the momentum. Most of our communication takes place in email or Twitter or Facebook (sometimes through Twitter) and the challenge Google will have to overcome - and it's a big one - is inertia. How to get large swathes of people to transfer their time and attention to Wave when they already have to check email, Twitter, Yammer etc.
Obviously, it's early days and very few people are allowed to use the system. The acid test will be when it is open to all. When Google launched Gmail the product took off very rapidly because it was a superior email client, and crucially because it was interoperable with what had gone before.
Wave however is not interoperable with email. Can Google change the habits of the populace at large? I'm not so sure.