I have a Dyson, but I think to fully understand you need some back story. First I am cheap.

I have never met a nickel I did not like, I shop for needs not wants. This is not due to lack of funds, husband and I have high paying jobs, and no bills.
About 4 years ago, I went over to the neighbors house. Now this is a neighbor whom I would often visit, and to me their house always had a "stinky pet smell" not pet deposits, just general funky not clean smell. The neighbors have 3 dogs, two of which are very large. So this particular afternoon I walk in and it is clean and refreshing, the wife's mother purchased her a Dyson.
Since I could not truly trust that the Dyson solved the household stink problem I embarked on my own Dyson experiment. At the time of the Dyson discovery, I had a good vacuum that I thought worked well. We have two cats, but our house has never had a stinky pet smell. I vacuum 2-3 times a week, I enjoy a very clean home. So I decided I would vacuum my home with my "own" vacuum, and then wait 15 minutes; at that point I would use the Dyson. My plan was that if I did not see an appreciable difference, I would return the Dyson (discussed in advance with the BBB manager). So once I plugged in the Dyson, I did not even finish my living room before the canister was full--I had just vacuumed but the Dyson was that much better. Needless to say my Dyson is 4 years old, and I am in love.

I can not imagine ever using another vacuum, for the same reasons others have posted.