I installed the new Firestorm viewer tonight and it is working just fine. It seems to be faster than the older version, and it has more features which brings it up to parity with the official Linden Lab viewer. And Carl is right in his comment below - it does take better photographs!
A clean install is always recommended. I use Revo Uninstaller which is a freeware program -- in the "level 4" which is the maximum uninstall mode to make sure my registry items are cleaned up. You can download Revo Uninstaller here:
You can download the latest version of the Firestorm Viewer, 4.7.3 here:
Inara Pey has a nice review of the new viewer here: