I bought a Dahon Jetstream P8 (2006) for pretty much the same reasons as you, though my commute is 8 miles each way. So far, I've only used it on the tube (Northern Line) once -- when I picked it up from the bike shop in Waterloo. Since then, I've enjoyed riding it too much to think about getting the tube for all or part of a journey -- and the weather has been great since.

The Jetstream is a great bike that rides really well (it's comfortable, too!) and has a good gear range (Muswell Hill is a killer...). It also seems to have few folder compromises and rides much like a normal bike. That said, the fold isn't particularly small and it makes an awkward package. I had to use velcro tape to hold the wheels together when I last carried it on the tube, and it's difficult to carry without a bag -- the fold isn't flat and there's no easy way to hold it. It's also tricky (but not impossible) to fold so that it can be pushed along on one wheel.

As a folder for last-resort train/tube use (puncture, sudden foul weather, unexpected inebriation...), it's great, but there's no way I could recommend it to someone who wants to carry a folder on the underground every day. The 16in wheel Dahon's may be better, but the 20 inchers are just too bulky.

Edit: Oh and check with your employer to see if they support the Ride to Work scheme -- you get a bike for half price!