Some of us have played around with the unofficial things which have been ok, but not so easy to use; however now we get the official integration and it sure is slick.
They seem to have a pretty good range of diversity as well. The two that I have settled on using right now are the google voice and gmail as default mail client extensions. There seem to be a few great ones for web developers as well to bring up to speed with the development helpfulness of firefox.
So far my limited experience with them has been really good. While the helpfulness of any given extension depends on its developer, the interface to use them seems almost perfect. They have a very fast download time, almost instantaneous install and uninstall, and so far have been unimposing and have not seems to affect my browser speed.