i've sold appliances for about two years now, so maybe i can be of some help.
the two companies that make these units (most commonly - there are other smaller brands that can be found at specialty stores) are KitchenAid and Fischer-Piykel (sp? my store doesn't carry the brand. it's most commonly found at Lowes). we have a display of the KitchenAid unit in my store, and people seem to like it.
the main advantage is that you have two independent dishwashers in the space that you normally have one. for instance, if you were entertaining at your house, you could start a load of glasses on the top washer, and when you started to run out again, start the bottom washer. in a traditional dishwasher, you can run a top-rack cycle, but you'd still have to wait until it was done to start another one, if that makes sense. you can, essentially, run the top 'rack' and bottom 'rack' on different schedules, if you choose.
also, it's more energy efficient to not run an entire cycle just for a few things. you can run a half cycle in the regular dishwasher, but because of the open space, you're STILL going to waste water that way. the two dishwashers are independent, so you use exactly half if you only run one of the drawers - there's no waste.
the drawers are pretty deep, so while it may occassionally be difficult to do something very large, there shouldn't be very much decreased functionality. also, i find that in the drawer models, the racks are often a lot easier to manipulate into different configurations, which gives you more flexibility.
i'd recommend the kitchenaid model, just because i have more experience with it. the double drawer kitchenaid and double drawer fischer piykel both come out to $1500 in stainless steel when all is said done (for the kitchenaid, the dishwasher itself is $1300 and then you pay $200 for the exterior panel kit for the color you want, the FP is just $1500 as a unit).
if you have any specific questions, feel free to PM me or ask here
the ones with a separate cutlery drawer at the very top instead of the stupid basket at the bottom that my current one has. miele have great dishwashers (well everything miele is great!).
well only when they aren't looking :shame:
