The few aftermarket radios I've seen that have a "dimming" feature only kinda sorta work...basically, when they get any sort of signal from the instrument light wire, they go to a lower brightness level. However, it's only slightly dimmer (and IMHO too bright), and you can't alter the intensity with the rheostat like you can with the factory radio. Not very impressive IMHO, particularly considering that it should be easy to implement a true dimming system.
OTOH, having dealt with a number of aftermarket radios, I have to say that I've never found one that I was impressed with from an interface standpoint...it's almost like they go out of their way to make them difficult to perform relatively simple tasks with (such as adjusting the clock, adjusting bass/treble levels, or turning off that stupid screensaver
), compared to factory radios. The only reason I run an aftermarket radio in my truck is that I want CD/mp3 player capability, and I'm taking extraordinary steps to keep the factory radio in my car...