Ask several of the staff which vacuum we recommend and you'll undoubtedly hear the brand Miele. Both myself and Aaron over at ATNY have gushed about the tested and proven performance of the Miele vacs (the Callisto put out a particle output of "0" according to our tests). And now there's yet one more feature Miele brings to the table, and a relevant one considering these high energy cost times: adjustable wattage use!
The Miele 1300 W EcoLine Green model is notable for the option of adjusting the power use of their vacuum from 1300 W to 2200 W, allowing you to switch over a lower energy use setting when full power isn't necessary (like when cleaning smooth and fairly clean hard flooring opposed to carpets or deep pile rugs). Other features of the EcoLine Green includes the Super Air Clean filter (which can be replaced by an Active HEPA filter, which we recommend), six-stage rotary selector for different suction power for different tasks, a "Silence" mode for when you want to clean without bothering others, and a full assortment of on-board accessories that hide within the vac in the same way as the Callisto model.