No problems with my aluminum fixed Swift. Not only is the Swift frame plenty stiff for fixed-gear use, but it's also the easiest to convert with it's track-style rear fork ends.

San Francisco, with it's terrible pavement and notorious hills, is the worst place to ride a 20"-wheeled bike, let alone one that is fixed-gear (anyone who has ridden Geary street end-to-end knows what I'm talking about). In spite of the repeated punishment I continually inflict on my Swift on a daily basis, the frame has held-up extremely well. I'm really happy with my bike.