I have several SL viewers on my desktop: the main viewer (1.18.5(3) at the time of writing), Windlight, OnRez and I had the beta grid viewer as well. It's sometimes useful to have two or three viewers: I can be logged in as myself with one of them, and use an alt account (or my wife's) to login with the other. In doing so, I've noticed a couple of weird things with regards to the OnRez viewer.
For one, it's using much more memory (RAM) and CPU than the main viewer or even Windlight, sometimes using about double of what those other clients use. I did a little experiment, where I put two avatars next to another at the same location. I did nothing else but monitoring the CPU and RAM. As for memory, right now SL main viewer is using 145 MB RAM and holding, OnRez is using 375 and rising. As to CPU, OnRez is using anywhere between the same and double the amount of CPU the main viewer uses - but never less. (all figures according to Windows task manager.)
In a later experiment, I changed some values for OnRez which differed from the main viewer's settings, most notably the cache size, and this improved things a bit; but still OnRez used much more memory than the main viewer - I'd say about half.
And, OnRez is slower to load the environment once you're logged in. I used to think that was because I am starting it as a second client and my laptop must be very busy, but I also noticed this with only OnRez loaded.
Finally, I stood side by side with my wife at our plot of land at Timandra; she was using the OnRez client, I was using the SL Main viewer. The main viewer provided a much better SL experience in terms of visuals. It just plain looked better, at exact the same time and location!
I'm quite happy with what OnRez is trying to do with their viewer, but there's a few issues they need to solve in future releases, in my humble opinion.