My company has been using both Vue and Carrara for a few years now and they're both great. Vue's Ecosystem creation is unmatched and it renders beautifully, but Carrara is so much more versatile that it's become our weapon of choice.
I strongly disagree with Shonner's claim that Carrara is "very slow." I can get a comparable render out of C6 in less than a quarter of the time it takes Vue and less thant half the time Mental Ray would take.
The nice thing about Carrara is the amount of control it gives you over your render settings. Of course like Mark said, you have to learn the program.
I know we all have different standards, but I've always found Carrara to be very stable and light on resources as well. I have it installed on a few older machines here at the house as well as on my 4 year old laptop and it runs just as smooth as silk :)