I've successfully converted 99% quality image which became 4.4 times smaller in size (identify shows 72% quality) after that I've converted same original image to 72% quality with imagemagick and I've got similar compression. Artifacts are look similar to jpegmini algorithm.

More interesting would be blind comparison experiment with with imagemagick compressed images with similar compression ratio. I'd be surprised if they could get 25-50% better compression with similar quality images.