There are a lot of very nice features to this and I'd say it works quite well.  There are down sides too, but it's only been live for like 48 hours.  Login is very easy, the design intuitive and quite functional.  Couldn't be simpler to install.  There isn't sound or any other way to notify you that you have got a new message except for a visual cue you'll only see if you are looking at the box in the same tab, etc.  Also a bummer that usernames are limited to 7 characters.

In case you are wondering, it does show all the users online on any site, but the ones on the site you are on are displayed first.  I like it.  One thing I'm not sure about is whether it will work if I put it in my personal blog's sidebar.  Presumably it only tells me if users are on the same URL, not within the same domain?  I wonder.  I'll have to test that out next.

Thanks to Brian Benzinger of SolutionWatch.com who helped me test this out.  He's got a review of a related product called Mabber as well.  Wonder if Mobber got the URL first and Mabber got the product to the public first?  One way or the other, Mobber.com looks like a winner.  Any readers here care to join me in a little chat to try it out?  I might even notice that you sent me a message.