A couple of minutes ago, I've downloaded and installed the OnRez viewer. I must say, I'm inpressed with what I've seen so far. For instance, it looks cleaner and more tight when compared to the normal SL client (wich I will call the LL client, as of now). At first sight, OnRez looks much more intuitive than the LL client as well.
I've tried it on Second Life, and on my OpenSim install, and both seem to work ok. At that time, OnRez was using around 120 MB of RAM. Then, I started the LL client in order to compare the two, both usage and memory wise. All over sudden, the memory usage for OnRez dropped to 16 MB, and the waves turned funky.. after I quit the LL client, and restarted OnRez, all was back to normal.
Might have been a bug, though, because when restarting OnRez, I got an upgrade notification for a newer version! That was fast :-)
A couple of screenshots of my OnRez viewer can be found here.