The name says it all! Rated 4 out of 5 stars

This add-on provides a set of tasks that can be initiated via keyboard control. These tasks include searching on multiple search engines, weather/temp, calc, tab counts, e-mail check, twitter posting etc. all without having to move to new tabs.
The attractive feature is you can customize it by adding set of custom commands created by developers. For instance you can add custom search engines to ubiquity, a local one from my country enables tracking a train and a lot more.
Suggestion would be to make it more adaptive. For instance if some other add-on is installed its list of commands could be made accessible via ubiquity. This project deserves a lot more respect and active development.
In short this add-on is one of the reasons i would say firefox is a lot cooler than chrome. For all keyboard shortcut maniacs this one is a must have.

This review is for a previous version of the add-on (0.2.5.1-signed). 

technical evaluation

"if some other add-on is installed its list of commands could be made accessible via ubiquity"
If two windows have one label and each label has two alternatives, the user goes to shell, runs "firefox -cli", sees:
- what window do you want?
- what alternative do you want?
If what alternative belongs to what label is clear, this works.
So it may be possible.

Also, if I go to a subwindow without going to a window and the window says give the subwindow a spotlight, the subwindow wouldn't have the spotlight, because I didn't go to the window.
I would have to make the window invisible, automatically go to the invisible window, automatically click in the invisible window to go to the subwindow.
So it would be slow.

Excellent Idea Rated 4 out of 5 stars

The idea is exactly what I have wanted, as I find the Ubiquity interface clunky.
However, it appears to conflict with Ubuntu's Global Menu Bar integration, which I use.
A couple of features that would be nice would be the ability to open things in the current tab (currently they open in a new tab) and the ability to see the Ubiquity suggestions (similarly to how history/bookmark suggestions are shown.
Great addon.

This review is for a previous version of the add-on (0.2.2.1-signed).