I like to keep my cars as stock as I can with only minor mods for performance (if any), style and audio. I don't like the look of aftermarket radios and will spend the extra money to keep that stock look. I did purchase and install an 08 (thanks Pooners!) Monsoon head unit w/ an AUX port. Plus, I upgraded the speakers to Kenwood and added an external amp to add power the speakers. It sounds really nice.
The newer Monsoon head unit, IMO, is the way to go esp. if you think you might sell the car later. Although we want to believe differently after market radios do not add value to your car. As a matter of fact I would NEVER buy a car that doesn't have the stock radio.
There are aftermarket head units that carry over most of your cars information but how much, I don't know.
My money would be on a newer Monsoon head unit, but I know others here do not share my view, maybe they will post up?
None, if I want a faster car I'll buy one.