I have always heard that Kirby makes a pretty good vacuum. I have seen people with Kirbys that last for years (30+)
I think the Kirby are very expensive, from what I have heard.
Yes, they are great, and yes, they are expensive. The are intended for people who run hotels and the likes. However, you should never need to buy another vacuum cleaner again.
They are heavy, so somewhere with stairs will need careful consideration.
I love mine, but I didn't pay for it (Got one when my dad sold his hotel, and was told to take the "Old Hoovers" if he wanted them. At £1300 each, for free, less than a year old, still under warranty, he took em). Think of it like a decent Model M compared to a $5 rubber dome. The cost is initially high, but factor that over it's projected lifetime.
Dyson are good, if you're mechanically minded. Spares are plentiful. I still have a DC01 here, good as new. Needed a few belts and filters, but they are consumables.