It claims to be better for SEO than Wordpress, and it automatically optimizes all images so the site loads lightning fast.
There are dozens of beautiful pre-made templates that are proven to convert well (or you can build your own custom designs). It's also designed to make it easy to do split testing.
If you build sites for clients, it is much easier for them to use than Wordpress or Joomla.
It's early days yet, I've only just got the software. But I'll post updates after I've used it for a while and have some tangible results.
But so far I'm very excited.
You can find out more at TopSiteToGo.