I don't regret it. I really enjoy it. I've always wanted to be a road bicyclist, but being Asian and of shorter stature, it never really felt comfortable to buy a road bike. Maybe road bikes just aren't comfortable for anybody and I'm overthinking it. But I felt at ease on a Brompton.
The smaller wheels do make it much more maneuverable than a standard bike and I can easily weave through cars that are sitting at a light but of course it's pretty dangerous. And I can take the bike easily onto trains and public transit without taking much space.
Lugging it up a flight of stairs with my work bag can be a bit taxing, but I see it as exercise and mostly only do it when the escalators/elevators are out of service.
Also, don't knock the A Line. My wife has one. She doesn't ride it as much as I ride my C but we throw them in the trunk and ride on scenic trails in the area. I take it whenever my C Line is out of commission and it's pretty nice still because it's lighter than my C Line by maybe 5 lbs. It doesn't sound like a lot but when you're carrying it, you will feel the difference.