Dahon and Tern occupy the sort of middle ground between BSO and Brompton. They have some good bikes in their ranges, but also some absolute stinkers at the bottom end.
I've owned a Dahon Vybe and sold it after a year, a truly terrible cheap bike. I've now owned a Tern Link Uno for the last 9 years and it's a great bike. I commute daily on it and it's really served me well. Whilst the single speed Link Uno is no longer available, the Link range is still going with a mix of internal hub gears or derailleurs.
There are a few thoughts to share. Firstly the frame hinge joints on both Dahon and Tern are rated at 100kg which isn't a lot when you factor in full Winter clothing and luggage etc.
I personally would go for an internal geared hub rather than derailleurs. Derailleurs sit to close to the ground on these bikes and need constant fettling and cleaning to keep them operating.
Folding, they fold as quick as a Brompton once your used to them and fold size is not that excessive. Mine easily fits into the boot of my car.
Overall though, if you don't need the really small fold size of a Brompton then as long as you buy carefully Terns and Dahons make good alternatives.