Ok so it's v 0.66112 and it does have some bugs. Mainly, the text in the generated PDFs might sometimes not display properly in old (10.6 or older) versions of Preview or in other non-Acrobat viewers. And you can't customize the compression or tweak any parameters.
But on the other hand, nothing comes close in the size vs quality equation. All those other PDF compressors, squeezers, shrinkers, optimizers, etc. work "blind", removing non-printing data and recompressing pictures but never actually rendering the file. If your original file was created very efficiently with non-cropped pictures and without much picture overlaps or image transparencies, fine. But otherwise, if your PDF file (i.e. Illustrator export) has a lot of those things, a 200Mb brochure reduced by PDFLight to just 1Mb looks almost as good as the original, vs. a 3Mb version produced by other tools in which the extreme downsampling and high jpeg compression make it look like crap. A 10-15 Mb version produced by other tools is about the same quality as the 1Mb version from PDFLight.
PDFLight achieves this because inside there's actually a PDF render engine (a RIP by old terminology) which leaves only the actual visible portions of images, and flattens them all for maximum efficiency before compressing. While keeping text and non-transparent vectors as such.
I really think when v1.0 comes out and all bugs are squashed, there would be no reason to use anything else.