We bought a Dyson on sale for I think $350 or $400. I was always skeptical too. But it definitely picks up a ton more dirt than our old $100 vacuum. We only have to vacuum about once per month now, and time is money. It's very good with the pet hair too, which shortens the vacuum time each month. I don't know about reliability, but my guess is the Dyson will also outlive our cheaper vacuum.
You could probably also make the argument that a house cleaner, at $75 a week, is worth it if your time is valuable enough. In a month or two, you've blown through your vacuum budget, but they do a lot of extra stuff, and, depending on your situation, the money might be worth the saved time.
Now, if you have a Dyson and a house cleaner, then you're probably not making the best financial choices.