To most digital sculptors Renderman makes them think of one thing....fast rendering of displacement maps. So it may surprise you to learn that for me that's not the thing I like most about it. My favourite feature in the pro server / Renderman for Maya combo I'm using is the Renderman relighting controls. As I mentioned in my last review lighting a scene can be a pain sometimes if you're not getting the right feedback from your viewport. Renderman has a nice way around this by bypassing the viewport altogether if you wish and relight fairly fast. This mean you get to see EXACTLY what you're going to get at render time and not a viewport approximation.
So the ability to change your lights settings, colours etc is a great boon, although there are certain times when you will need to rebuild your shadow map. One thing would say is that for a scene such as the cathedral you really are going to have to wait a fair old bit. Because even though there is no displacement at all in the cathedral render in either Renderman or Mental ray version (its basically a game model on steroids), when your pushing that much information around even Renderman is going to take a while to catch its breath.
When Pixar were nice enough to let me have a free copy of the latest Renderman for a while I have to admit coming from non Renderman compliant render engines it was a culture shock. Renderman is at its best when arguably it's at its least 'intuitive' to a non coder. If you are prepared to write your own shaders, you can really make it do tricks as can be seen on virtually 80% of all major Hollywood films. If you don't then you are I feel almost gutting th heart out of the render engine in some ways.
While I was using Renderman for Maya, I was also using Renderman pro server alongside some apps of my own that aren't available publicly such as Exodus. This enabled me to take a look at pro server in a less modicodelled environment and use it 'in the raw (and by that I mean without a UI...not naked lol).
I did find both a bit of a pain to set up, although some of that was due to my extremely flaky development machine which is renowned for throwing errors no other machine on planet earth seems to. The one area I would like to see improved is the installer as messing around with environment variables if something goes wrong is no one's idea of fun! But again Pixar came to the rescue and managed to get me up and running in no time with some of the best customer support I've seen anywhere .
So how did I cope with having to use Renderman in Maya when I am mainly used to 3dmax and as a Mental ray user? I'm not going to lie and say it was all very easy , there was a lot of information I needed to take in, in what was a very short time. If you're looking to pick up Renderman and use it in a day or two to its best from nothing then you'd going to be mistaken.
Renderman needs time to be learned and does to some degree at least ask (rather than demand) you to do some coding to get the very best out of it. It's not the fastest ray tracer out there and physically accurate lighting and shading out of the box while not impossible is far harder to achieve than mental ray. But comparing Renderman and Mental ray, although both are render engines is like comparing a fish to an apple. (While both apples and fish are good at being apples and fish, but you wouldn't want a fish on your fruit salad.)
There is no 'best render engine' the best render engine is the one you have and the one you can get the results you require out of it the easiest. I wouldn't call Renderman a good engine for a beginner to learn unless they have a lot of time and are prepared to go quite deeply into rendering theory. Its only as good as the artist behind it and not a magic wand. Overall I like Renderman and can see m using it for jobs that fit it best, the same as I use mental ray for jobs that fit that best. (Plus a little secret...no one ever said you can't mix and match passes from both engines.)
Renderman's renowned fast displacement times are not ironically as big a deal for me as you would think, as that's a very small piece of a larger puzzle that is your rendered frame. But a nice plus is that Renderman can render a Mudbox vector displacement map without any special shaders or complex set up. It's a tried and production tested industry standard render engine an as you would expect it does what it says on the tin.