I have been using it for a while, and it is a real convenience feature. I cannot tell you how many times I was saved by it.
Do you think it is safe?
Does it have access to the unprotected password list? In the same lines, how easy is it for other Firefox addons to access the list of passwords? If it is easy, and there is standard API for it, then in principle other addons can also call home with the passwords, isn't it?