I have an RB, a Domke F6 (a smaller F2) and just bought a Retrospective 30.
The Domke F6 is a very nice bag and I can actually put the RB in there if I take all the inserts out but then it has no padding on the sides which means it rubs against me with all the knobs and metal bits. Not nice.
The Think tank is great, has padding but not too much to make it unnecessarily bulky and will fit the RB vertical (so it then fits another 2-3 lenses and bits and bobs) or horizontal with a prism on top or lying on the side with the grip. I think the Retrospective 20 will fit the RZ vertical but not horizontal. If it did fit it horizontal then you're kind of wasting the bag as it is deeper than longer, whereas the 30 is longer and less deep (but still fits the RB with prism and still has some space on top).
I took the bag out on its first outing, around 4 hours walking around with RB, 90mm, 180mm, grip, extra back, lightmeter and film. It was comfortable and very easy to use. I particularly like the "silencers" on the velcro (they hide the velcro so the flap just rests on the bag). Makes it very easy to open/close the bag.