As Internet is my first tool to communicate, I was first quite disappointed because I couldn't reach a lot of my friends. This is the reason why it took me so long to write an article about the features and usability of Wave.
Google made a great job in keeping the layout quite close to what we would expect for e-mails. There is the folders menu, your contacts, the lists of messages and a window to read the current wave. The whole thing is very similar to Outlook or Thunderbird. So, it's very easy to understand the principles or to use it the same way as a simple e-mail account.
But once you use Wave, you cannot miss the fact that it's different from e-mail. You can structure your messages very easily so that anyone can follow your thoughts. The only issue is that it takes ages to organize the thread in a clever way. But then you can add anyone to participate and with the playback option you can easily understand what was discussed.
What is really disturbing is the fact that people can see what you write in real time. It decreases the ease of use of the service since Wave is quite slow to respond when you type (in order to be able process the information and send it to the participants of the Wave). Beside, sometimes it's really comfortable to be able to read what you write before sending it (you might be willing to smooth your criticisms, correct your mistakes...). Yet, I like the real time interaction real time monitoring provides. I feel like it could be a great time saver if people adopt this way of writing and thinking in the same time.
There are many bots that allow you to do many different tasks. I tried to sync Wave with Twitter successfully. There's just no point of doing it... You're supposed to be able to sync Wave with your blog, but once again I can't see the point unless for a collaborative post. Other bots enable you to write mathematical formulas or to translate your message in different languages.
Finally, many add-ons are created which might be the real advantage of Wave. For example, Google Gears allows you to use Wave as a part of your computer (you can dreg and drop contents for example). But there are other examples that are more impressive, like Gravity (I didn't try it but it emphasizes the possibilities of Wave).
As a conclusion, I would say the Google Wave is definitely a great improvement in on-line communications. Yet there are still some features that could be better (which is completely normal as that's only a first try). The huge time of response is a great problem for me.
Once understood and adopted by the mass, it could definitely replace e-mails. But could everyone use it, or is it just a tool for the on-line elite?