I did the Ride London-Surrey 100 on a Brompton the other day along with thousands more road bikes and a few other Brommies, and I can guarantee I had more fun than the roadies, more kudos from fellow riders and more cheers from the side of the road. That's priceless! Made what would've otherwise felt like a miserable winter training ride into a great experience. Just wouldn't have been the same on a similar value road bike.
But honestly, having had a hard crash on one, even though it could do with a new frame, as it's slightly bent, my first brompton M2L is still perfectly ridable after some headset adjustments and slight front wheel truing after going into the side of a car at 20mph that left me with a cast on each arm (wrist and hand breakages). They are really well made. My newer M6L has done 6,000km in less than a year and has only needed new front break blocks, chain and sprockets so far. All pretty cheap replacements if you're doing them yourself (£35?). Oh, and about £30 quid some some kojaks for summer, and maybe winter. Rest of the bike still looks (after a clean) and rides like it's new. Well, if you don't look at the slightly scuffed SPD pedals. There have been a couple incidents. I'll still probably replace the frame on my M2L, as it does bounce a little more than it should at higher cadences.