Rated 3 out of 5 stars
I find this add-on to be brilliant except for one critical flaw. Most of the time, on one particular computer I use, restarting firefox causes the address bar to stop working entirely. This means I have to disable the add-on and re-enable it to restore functionality.
Other than that, I love the fact this is a bootstrapping add-on, as it makes updating and installing and disabling, etc. completely painless. I also love being able to do searches for multiple services from the same bar with ease. It does also need the option to add search engines though.
If it weren't for the major issue I raised, this would be a 5/5 for me.