Brompton – fold very small, fairly light and easy to carry, ride reasonably well.
Mezzo – Awful ride, Brakes are rubbish, lots of bits to go wrong.
Dahon – Ride very well, fold is bulky and it falls over, heavy and awkward to carry.
Airnimal – You have to take the front wheel off to fold, enough said.
The reason bromptons are so popular is that they offer by far the best allround ability.
(I own a Dahon speed and have commuted via train into london for the past 3 years, ive ridden all of the bikes above.)