Rated 4 out of 5 stars
Without it, I'd never have written an extension. With it, it's as simple as can be.
Now with upcoming built-in publishing/updating to AMO (this site here)! All kudos to dandonkulous (see below):
The push-to-AMO feature is now in the product and will be blogged about when we release the 1.0 final version of the Add-on Builder site in mid-February 2012 (about 3 weeks!)