Personally, I'm holding out for a Dyson Animal ~ $549.00 at Target; the basic Dyson Ball is $399.00. I've been wondering myself if the extra $150 is necessary. Either way, I figure I'll never need to buy another vacuum cleaner in my life, unless I lose it for some reason.
The thing about vacuum cleaners that's changed in the past decade or so is that most of them are made with plastic motors, not metal. Dyson is the only one I know of that still makes their motors out of metal. There could be others out there, but I haven't done much research. I can attest to the difference in motor lastability: I have a 21 year old Hoover Port-a-Power with a metal motor & it's going strong. I know people who have burned out a couple of vacuums in that time.
Brands aside, look for a metal motor with a lot of power and you probably can't go wrong.