SezWho is shutting down. JS-Kit is keeping the service open until March 31, 2009. After that time, anyone using SezWho will need to sign up for JS-Kit. JS-Kit is the world's largest distributed social network, connecting over 600,000 publishers with light weight, cross domain services, including comments, ratings, and polls.
I tried SezWho when it was first suggested by Entrecard. It slowed down my blogs, so I removed it. Perhaps, the service will be improved by this change. I would wait before I rush into it. However, if you are an existing user, you should probably sign up for the JS-Kit, even if you do not use it at first.