Well Scott, Liquid works with all REYES tools where MTOR does not. I know very few pipeline people (TDs in general) that would opt for MTOR over Liquid. That said, everyone always has a favorite.
MentalRay sure does get a lot of talking up, I was involved with the founders of it to license me a thousand copies for a farm about 2 years ago, but when I got right down to the real world testing it fell apart big time. They offered all kinds of support to get it all working, but I just felt that REYES was the thing to stick with. Sofar I haven't been disappointed, besides it's easy to take the simple MR files/shaders and convert for REYES rendering.
Of course I will admit I'm biased, as I did work for that little company with the string of hits, oh so long ago when they were practically unknown.
Joojaa, I'm glad it works for you. I find it interesting though that you think the interface is weird in Liquid. It was for me as I really hate Maya, but those who know Maya seem to feel it's just right. Yes I use Maya - as a container, what it's best at. Previously I utilized custom tools for about 6 years, before accepting Liquid on recommendation by a friend.
I also don't see the issue with hair and volumetrics either, but perhaps I was just lucky in production. Either way, grab a tool you like and rock on!