Why is the average car with an average sound system so damn revealing about things (like bass) in your mix?
IMHO I don't think that checking a mix in the average car has anything to do with a car being some kind of "reference monitor". I bet that you can take a mix to 10 different car systems and then come away with 10 different lists of changes to be made.
The reason why "checking in your car" works is that you just get out of the box you call a studio and get to hear the mix in a totally different environment. Then it doesn't matter if that environment is a car, a PA or a mastering studio.
That said... I have great fun with my MixCubes!
I love using them when tracking things like vocals 'cause it's much easier to pinpoint problems like pitch and delivery while hearing mostly midrange. They're also great for overseeing everything else. It's just easier to diffrentiate the quality of different takes on them since it's such a boiled down format. If something has even the smallest hint of uncertainty to it, be it a guitar track, drums, bass... whatever, the problems are going to show up like it would on a system where sound quality in itself isn't a key component. (radio/tv/computer/mp3....)
I can't bring myself to do a full mix on them though since I'm used to judge frequency relationships by hearing most frequencies at the same time. I'd probably end up with mixes that have way too much 15K>, way too much <100Hz and a hole around 3K-6K. Be interesting to know how people who DO make full mixes on them actually work, 'cause after 2 hours of listening to them you just can't listen to anything else for 2-3 hours while your ears re-adjust themselves.