Bromptons are very overpriced, the gear system is quite clever, but there is no reason a basic Brompton should cost more than £400 and anything over £700 is ridiculous, these days putting a hinge in the middle of a frame is not the cutting-edge of engineering technology but it seems to double or triple the price of folding bikes. It's not that Bromptons are great, the problem is that cheaper folding bikes are poor by comparison.
Having just purchased a Brompton it soon becomes clear why they are more than a bike with a hinge and why they command a high price.
It's not a high volume product, it's hand made in the UK, probably 99% of the parts and components are specifically made and designed for it. The fold is unique and incredibly well thought out, way better than any other - all the dirty gubbins on the inside of the fold, the way it locks together, the way it stands by flicking the rear wheel under, you can use it as a shopping cart yadda yadda yadda. As a bike it's very versatile, with use you soon realise it's capabilities, it's ease to travel with, it's load-lugging capabilities. It copes with smooth and roughish terrain. It's a lot of fun to ride, comfortable over distances and surprisingly quick. It has charm.
Other folders are good too, ride well enough, but If other folders were as good, they'd be charging more for them, but they're not.