I agree with all the positive things written here about Royal Render. I really enjoyed working with it.
I have no experience on how it scales up with bigger farms/job loads though, I only used it at a very small studio/farm.
And full Linux support would be a very cool thing, I am sure that would attract some bigger players and Linux based studios.
GridEngine is great and handles a crazy amount of jobs (185k jobs per day.... wow!) easily but I guess it would be overkill for smaller shops.
http://gridengine.info/pages/profile-DNA-Productions
http://gridengine.sunsource.net/Cheers,
Axel
Rising Sun Pictures