I'm really enjoying my new Brompton M3L even though I have just a few miles on it. It is a solid riding bike. For a lot of new bikes, I always make adjustments after an initial riding period. Adjust the brakes, tighten something...not with this brompton (at least not yet). The only adjustment I've done is for fit. I moved the handlebars slightly towards me. The suspension, basically a rubber circle in the rear triangle, is perfect for my weight. Not too stiff and not too squishy.
Anyway, let me report on some upgrades/modifications. First, I replaced the stock brompton seat, which was hard and uncomfortable, with a Bell MTB bike seat. Ok, I bought this seat at walmart, but it has served me well on other bikes. It has an anatomical dimple/hole and no springs for suspension. I was kinda worried that I would have to invest in a pentaclip, which promises to make every saddle compatible with the brompton seat tube. As I mentioned earlier, Brompton accessories tend to be pricey. I didn't need to get the pentaclip. The Bell seat came with a saddle adapter that fit perfectly on the seat tube. These adapter clamps are also available at your LBS for about $2-3. They look like this:
Second, I ordered and installed the Brompton front carry block ($30), which allows you to fit a number of bags to the front of the brompton. I followed the instructions from Channell Wasson (see:). Channell is the top dealer (perhaps the only one?) for Bromptons on the west coast. Basically, you just have to tighten it so you won't lose the bag as you're riding. For my first bag, I purchased the folding basket ($99). This is a open basket with a built in lower support bracket. I can use it to put my work bag there as well as groceries or other things I can carry around.
The best thing about the front carry block is that it is connected to the frame, not the handlebars. That way, you can carry as much stuff in your basket/bag and it won't interfere with your steering. Most of the front baskets sold nowadays are attached or clip on to the handlebars. This affects steering and can make your bike ride more twitchy or wobbly. It's a weird feeling at first because you steer to the left and right and the basket does not move with it.
Brompton takes a lot of heat for its expensive accessories, but I'm really impressed with the quality and workmanship of these two items. So far these two have been money well spent.