One of the wisdoms in web application development is "Release early and often."
Google and Twitter have both released software "tests" to select hundreds of thousands of users, both with the idea that there will be problems, but let people try them out, and then improve the software iteratively, based upon real-life user experience.
This is my first blush impression of these previews I've been privileged to explore this week.
Get on my Wave!
I've been trying Google Wave for this past week now. It's been a bit hard, since hardly anybody I know is on Google Wave, and of all the people I invited, only two have received invites so far. (I got 8 "invitations" that turned out actually to be "nominations" once sent. Sorry, Google, but invitations and nominations are different things.) So I've had only limited exposure to what Wave might offer. One on one, it's pretty much a glorified instant messenger.