Rated 5 out of 5 stars
I've never experienced lag like one user has.
It does not wipe any data from standard sessions (which if it did would be the fault of FF not this plugin).
Gives access to a large amount of preferences... like the ability to save cookies or passwords from the private session (though that IS less private, it is a convenience).
While Ctrl-Shift-P may give FF3.0+ quick access to private browsing, the options are not readily available without this (AFAIK).
I love this plugin. I can't imagine FF3.5 without it.
5 Stars: Does what it says on the tin.