The good points about my old Brompton.
The fold. I could fold or unfold my Brompton in seconds (at least I could when I first got it), and parking it was as simple as kicking the swingarm under the bike, no kickstand was necessary. It folded small enough to keep in the closet in my office.
The comfort. My Brompton had a good position for commuting; nice, and upright, and after adding a Brooks B17 and Ergon bar ends, it was even better. I discarded the stock pedals as soon as I got it home, and swapped on some MKS quick-release pedals, which were lighter, better looking, and worked better with my shoes.
Components. The shifter was plastic, but did the job. I had no trouble working with the 3 speed hub, having ridden "English Racers" when I was a kid. Having such experience, I knew how to make the adjustments to keep the shifter and hub in direct communication. The brakes were the same as the brakes on any other bike, they work and are adjusted the same way.
The bad things about my Brompton.
Serviceability. To do minor repairs you need to carry tools. I am a competent bike mechanic who has worked in a bike shop, and can do any bicycle-related repair or adjustment. I can change a rear flat on a Brompton in 10 or 15 minutes, provided I have a wrench to remove the axle nuts. But for those who are not mechanically-inclined, changing a rear flat is challenging. I can change a tire on a conventional bike with quick-release wheels in 5 minutes with no tools at all. The frame is mild steel, and in the places where the paint gets rubbed or scraped off, it will rust. The contact points between the frame joint and the clamp rusted on my Brompton, causing them to stick, which required a few kicks to open when I wanted to fold the bike. A little grease would keep it working for awhile, but eventually it would stick again, and people walking by would wonder why I was kicking the hell out of my bike.
Weight. Despite their small size, Bromptons are heavy. The frames are durable and sturdy, which is a different way of saying "heavy". The weight somewhat negates the benefit of the small fold.
Gearing. No matter what the flavor, 3 speed or 6 speed, the Brompton does not have the gearing for varying terrain. As a commuter in a flat city, or with rolling hills, the gearing is adequate. For people who live in areas where there are mountains, it is not. My old neighborhood was flat, as was my work commute. My new neighborhood has steep hills, too steep to ride a Brompton without dismounting and pushing. (I can climb, as a racer I climbed the Ventoux in France, and the Angliru in Spain).
Upgradeability. There is a large aftermarket for Bramptons, with a huge number of parts and accessories. Unfortunately, these parts are Brompton-specific, and are expensive. One can indeed upgrade the gearing on a Brompton, but doing so is expensive, and requires a fair amount of skill. Schlumpfs and Rolhoffs can be added, for the same price as buying another new bike, and these add to the already significant weight of a Brompton.
I would not buy another Brompton now. It is not suitable for the local terrain. I could modify one to work, for the cost of two other bikes which I could ride without modifications.
But if I lived in LA, New York, Miami, Paris, or London, I would get a Brompton, it suits the terrain in those places quite well.