Keep in mind you are asking people's Subjective views on Vray. For example, I used to use Podium and loved it, it was quick, it was simple, very easy to learn. Vray on the other hand, is very powerful (as noted not as powerful as it could be with missing features and all) but also a lot more complicated to get started. When I first tried Vray I hated it because my renderings with it were worse than what I could achieve in the same amount of time with Podium. Now that I have a better grasp of Vray I prefer it over Podium.
Oh and as far as I know, Vray for SU still only uses CPU except in cases of RT Rendering which is still buggy (correct me if I'm wrong on this).
And yes, you can use double sided material (transparent on the top, opaque on the bottom) and use that as a Faux ceiling for your units.
Rob