For the past few years, V-Ray has been the primary tool I've used to do renderings of my designs. Like most software, it has its ups and downs, but the interface is particularly frustrating and been a nightmare to accomplish even the most simple tasks.
I've heard good things about Mental Ray and Maxwell Render, but the idea of learning a new software package in the middle of a design competition seemed like a bad idea.
That said, after a few hours of playing around with Maxwell, I've managed to produce a better rendering than I typically would in V-Ray over a couple days. Needless to say, I'm a bit more enthusiastic about having some great renderings this time around.