Rated 5 out of 5 stars

The extension works as expected and the author is actively developing it at the time of this review. (speedy and friendly contact by e-mail) Fixes and enhancements are carried out fast. (e.g. Seamonkey is now officially supported, yay! \o/)

The colored representation of the identities when composing a message is really helpful after one one gets used to it and intuitively knows which color should be there at the moment of writing the email. There's some "automechanism" in your brain telling you "There is something wrong", when the borders of the compose window are red instead of green etc. because you chose the wrong identity.
This system of course has it's limits, because you won't be able to differentiate like more than a dozen colors without explicitely thinking about it, but most users won't have that many identities anyway...
(sidenote: the default colors seem to repeat themselves after every 4 identities, but this is no issue as they are freely configurable in the settings)

Most things can be configured the way you like (the colors, which buttons to change, if email-links should be handled too, etc.).
I'm only missing one thing with this extension: an option to get the original split-button-behaviour back so that it integrates into the drop-down-menu of the reply button etc. only and not disabling it's split-behaviour, if desired.

That would make a 4.5 rating from me. I'm deciding for 5 stars after getting a little bit frustrated with abandoned extensions and experiencing this very enthusiastic developer here.

This review is for a previous version of the add-on (1.3).