I commute on a Dahon Cadenza (the one with gears.. a fixy is so not me). It creaks a lot and I had to replace the front wheel which collapsed after about 3 months of use. Other than that, it’s a comfortable and reasonably fast ride in the traffic. It’s pretty smooth and responsive and the fold is useful when you get a flat in the rain (folded it fits snugly in the boot of a cab). Generally I am happy with it, mainly because it’s a generally comfortable ride most of the time, but I can bend over the bars and push it when I’m in the mood.
Update : Much as I would wish to be riding this great bike to work every day, it was stolen (from the office bike store!!!) some time ago. I now ride on my old and trusty Giant road bike.