<< What's the big deal with aluminum cases? They're a lot more expensive than the standard (steel?) variety. >>
They're lighter, they look cool, and they're better for modding (you can do more intricate work without losing as much structual integrity as a plastic case). Also, manufacturers that build aluminum cases tend to put a lot more thought, research, and good design into them than plastic ones (most likely for two reasons, first off the higher expectations of its intended market, and second, aluminum is easier to work with than plastic). I doubt you'll find a plastic and steel case that has a removable motherboard tray and backplate, has a removable 3.5" bay and hard drive cage, adjustable fan speeds, 11 total drive bays, with most major parts held together with thumbscrews (PC6x series).
I'd have to say Li-Li's are the best for value, but CoolerMaster has some better (or at the very least, a much larger variety) high end cases. Unfortunately, the CoolerMaster stuff is WAY TOO EXPENSIVE. Yeesh. So I'd recommend Li-Li, unless they somehow don't suit your needs and you can actually afford a CoolerMaster.