I've moved a couple of sites over Joomla (the rest will follow soon), I find the user interface a lot nicer and easier to use. Some of the end users who are not the most technical people in the world found it easier as well.
The default editor Tiny MCE is a hell of a lot better than HTML area 2.
Urls don't change so your listings won't be affected, (I don't have search engine friendly urls turned on most of the Mambo sites, I will be turning them on in Joomla).
The change over is pretty easy, I steped up to 4.5.2 then to Joomla.
Support for some components can be a problem though.