I don't have the plugin but I was very interested in it until I read about some of the problems people have had with it. Again, this is all hearsay but from what I gather so far:

(1) Since Xstream is particular to each render engine (in the case of Maya you can purchase Xstream for mental ray or the software renderer), my guess is that the trial only supports the software renderer, which is why your "mental ray specific" effects do not render. This is a major minus for me since I use Renderman For Maya (for which there is not yet support), but I've heard of 3D Max users wanting to use VRay, Brazil, etc.

(2) The wording at the website is ambiguous at best, but it appears that the plugin is basically Vue 5's render engine minus the browser, content , editors, et al. So the reason why your renders look look pretty bad is because, while Xstream takes its AA settings from the host program, you may want to tweak Xstream's render settings independently, which I'm sure is possible.

(3) Additionally I have heard that for $500 USD, the plug is not very stable (although I have had no difficulties with Infinite). I'm hoping someone with more experience and / or optimism will chime in but I believe that (a) many issues will be addressed in version two which will (b) be heavily influenced by ILM's use of the program and (c) the release of Vue 6. Until then I will wait patiently.