I won't say that the viewer is the best, but when I look at my panos I don't complain that much...
(clique on 'Visiter Htel MANSOUR EDDAHBI' to open the popup)
When you look at them full screen I found them actually not bad at all..
On a 17" they're quiet clean. On my 24" it's still very acceptable.
I'm using 4500x2250. files are between 1 & 1,5 MB.