An important thing to notice right away, is that NXT Solaris is (currently?) only available for Windows users, so keep that in mind. It does work on both 32-bit and 64-bit installations though.
These clients are designed for the people who don’t like to have their NXT client running in a browser window all the time, which isn’t too appealing in fairness.
It gives you more of a “graphical interface” feeling when using the NXT software, with all functionality still intact.
The NXT Solaris client also shows you the available NXT markets, such as BTER, DGex, Vircurex, etc.
Oh, and there are also different skins, or “styles” included with the NXT Solaris client. Some of us will find that extra valuable.
Overall, this looks like an amazing client, if you’re a Windows user. There is a Mac OSX wallet version in the works, but no official release date yet.