Car free since 1995
Join Date: Sep 2006
Bikes: Birdy Yellow, Birdy Monocoque
I've owned both a Mu SL and a Birdy. There is no comparison. Unless you are riding on an icy smooth surface, the Birdy is faster and it is a *lot* more comfy. I think the theory is that the suspension reduces rolling resistance, thus overcoming any inefficiency associated with suspension. If you ride the Birdy correctly, the suspension inefficiency isn't an issue b/c the rear triangle is isolated and the front only seems to move if you hit uneven surface.
I did 70 miles on the Mu one day and sold it the next week; it was much harsher than a road bike.
Still, the Mu is a light, fun bike. If you are doing a 10 mile daily commute on smooth surfaces, it's great. It's also easier to learn how to fold than a Birdy. I just wouldn't do any serious riding on the Mu.
Incidentally, I'm running the big thick Greenspeed Scorchers now on 349 wheels (non-Kevlar). I got a flat on the inagural ride, and the Stelvios seem more svelt. It could be that the bike already has suspension, but I'm not sure that they live up to their lower rolling resistance hype. The contact patch is large and the tires make more noise. But if you need to run a thick tire (e.g., on a Brompton), they are great. No reports on wet surfaces yet.