Be sure to drop by veteran mixer Phil Palmer's blog The Sound Cart. Recently, he posted a series of photos of his new cart that he had custom built.
Currently, I rent a beat-up old Magliner cart when I need one. It's cheap and it gets the job done, but it certainly isn't the most elegant of beasts. Down the road, when I'm (hopefully) doing more feature films, I'd like to put one together myself. Seeing photos of setups like Phil's makes it easier to visualize what you may need in the future.