SIFT seems like a pretty clever technique, but in reading the paper, I felt like they just pulled a bunch of values out of a hat. They claim invariance to all sorts of phenomena, like rotation and illumination, but they don't really present proofs. I hear that SIFT is popular among some groups, so it's probably worth something. Still, I'd like to have a better understanding of the underlying reasoning for such features, not just values of sigma.