To address some of the points and questions, try this thread:
http://autopia.org/forums/showthread.php?s=&threadid=53431
But in the meantime, DI and RO are great ways to reduce the levels of dissolved solids in your water, resulting in a spot-free finish--which ends up reducing any marring/scratching when you don't have to dry. Simple enough.
The units listed at the beginning of this thread I'm sure will do a great job, but they are a bit pricey. RO units will produce better water, albeit without the pressure, at roughly a similar price. If you can put the water up high (for a gravity feed) or add a booster pump, you'll get good pressure.
RO membranes will last much longer than either of those units--if you're worried about waste, imagine what it takes to produce the resins in those cartridges, as well as when you throw them out. If you try to recharge them (a process that takes very strong acids), consider that waste as well.
At least with RO, you can direct the waste water either down the drain or out onto the lawn/garden, whatever, so something else can use it.
Just my $.02...