I've heard a bit about this but it stayed in the background until recent changes in SL TOS/copyright raised some hackles in the blogosphere.
Of the grids listed, "Open Life" seemed the busiest so I opened an account. Worth a look just to compare with SL.
Fairly decent number of users here already. You use the SL client, just have to modify the command line arguments to point the viewer to a different grid.
I'm trying to get a sense of which direction the herd may start moving next if SL starts to lose its appeal. Only prob is you can't migrate anything (avatar, inventory) over, BUT I am not sure if that limitation applies in OpenSIM. That is, you may be able to migrate content from one open sim to another--by which I mean the way I can move my website from one server to another.
Looks like you can even set up and run your own grid. Once this catches on like running your own webserver, I predict land prices will bottom out and we'll approach the pricing levels for web space. The trick will then be relating grids to each other. Tesseract, anyone?
PS: I have not yet looked to see if/how this hooks in with MediaGrid and Immersive Education projects, which are also looking at open source sims.