I tried JSZip not too long ago for my side project and it was quite slow. I've been using http://gildas-lormeau.github.io/zip.js/ and I'm very happy with it. I hope to release my web app some time this week.
zip.js seems to be the better of the two. It uses typed arrays, so it's performant and I can zip large (140MB) files quickly and without hanging the browser.