I'm on Google+.
After a couple days of playing with it, I haven't quite identified what it is for, or at least how I'm going to use it differently from twitter or facebook, but so far I am generally impressed - it's easy, intuitive, and fast. It also allows you a level of selective privacy that - while possible to achieve - is very clunky on Facebook. It only took me 10 minutes on the web interface and another 10 minutes after downloading the Android app to figure out how it all worked. And Google+ is already far better integrated into the mobile user experience than Facebook is (though this is to be expected for a phone that runs Android).
The Huddle feature alone is worth the price of admission - which basically allows for group SMS. The Hangout feature is interesting as well - it is an amped up version of video-chatting, which uses your webcam and microphone to allow you to chat with up to 10 people at once. This is still limited by your own bandwidth - using my DSL connection at home, it was a bit choppy, but using a cable connection at a friend's place, the feed was relatively smooth. I think there's a lot of room for growth with Hangout, so while it isn't particularly useful to me at the moment, I think it certainly could be. Leveraged properly, the tools available in Google+ could be extremely useful to bloggers. For example, by increasing the number of users from 10, but allowing the "host" to set certain users as "audience" or "participant," Google Hangout could become something more like ustream; imagine bloggers being able to host live video podcasts, where viewers could participate via chat. With the proper tools to record the video, splice it together, and upload it to Youtube, Google Hangout could also allow people to create something similar to what bloggingheads does.
For the researcher, it is easy to imagine the convenience of being able to videoconference with collaborators, for example, while simultaneously editing a Google doc. Or private conversations conducted within a particular "circle" including only labmates or collaborators. If a member of a lab is out in the field, he or she could "call in" to a lab meeting using Hangout and share his or her latest findings, or to check in with the PI. I once gave a guest lecture to a class halfway across the country using Skype - while it was okay, there were still some technological limitations. If Google+ had been available at the time, those problems would have been trivial.
Beyond creative ways to use the tools, I remain unclear in terms of how exactly I'm going to use it on a day-to-day basis. To curate content (I do this with Google Reader already)? To engage with the wider sciblogosphere (I do this with twitter)? To keep up with old high school and college friends (I do this with facebook)? To be witty in more than 140 characters? With time, perhaps its use will become a bit clearer - after all, the product is only two days old. Given that Google+ is still in "field testing" operation, it will be interesting to see what they roll out in the coming days, weeks, and months.
Enrique Gutierrez's review (recommended)