I'll be the first to say I'm not exactly the housekeeper in the family, in fact I'm kind of a slob.
We've had a decent Sears vacuum for 5-7 years now. I clean it out regularly and replaced the beater brush bar once. It sure seemed to have good suction. As you would imagine with four kids and two dogs in and out of the house all day the carpet takes a beating (carpet is slated for removal as soon as we can afford to have the wood floors finished downstairs-we figured we'd have the carpet until the youngest was an adept walker). Admittedly it could probably stand a daily vacuuming but it's usually every other day by the time the Mrs. or I get around to it. My wife and 75% of my kids have allergy (dust and some pollen, not pets) or illness induced asthma (not severe, just very annoying) so we try to do it regularly even though it doesn't really look dirty. The Mrs. has pined for a Dyson for a while now but we've just never had the extra cash for one. Especially since I had our old one working so well.
Well, my mother decided that we needed a Dyson after she bought one for herself to replace the 25 yo Hoover she's had forever. My SIL bought one of the lesser Dyson models a few months ago and liked to rave about it to my wife just to stick it to her (they "only" have two kids and insist on buying lots of things to keep up with their friends and neighbors). So mom and my wife went to Kohl's to pick one up on sale and bought us whatever their one below the top of the line upright models, whichever one is in the $500 range (they ran $350-600).
Let me just say I was horrified by the amount of "stuff" that came out of the carpets that had received fairly regular vacuuming. Our living room is about 18'x12' and the Dyson about filled 1/2 of it's see-through canister with fine "stuff" that our seemingly well working Sears managed to miss. I wasn't overly excited at the prospect of dropping $500 on a freaking vacuum, there are lots of other things we could use the money for right now. But since mom was buying I wasn't going to get in the way and I'm glad I didn't.
I still think it's a lot of money however if you're in the market for a new one and if you or any family members suffer from allergies they might be worth checking out. We've only had it a week now so I can't speak for any reduction in allergic reactions but I can't see how taking an additional liter volumes-worth out of a "clean" carpet couldn't be helpful.