My experiences with JS.IO were very positive. Although the documentation was sorely lacking, Michael Carter (the author) was extremely helpful when I was getting started. Furthermore, my testing with crossbrowsertesting.com proved that JS.IO was very reliable across an incredible range of browsers. It was the most reliable comet library I tried. By the way, I was doing this cross-domain.
These days, Michael Carter has started a new project called Hookbox:
Hookbox is a Comet server and message queue that tightly integrates with your existing web application via web hooks and a REST interface.I blogged about Michael's talk at PyCon. Although Hookbox doesn't perfectly solve all my problems (it doesn't yet having clustering support), I'm extremely hopeful that this will turn out to be a great project!