I have an 04 Accord EX coupe, no XM, no nav. The system is quite good. Bass response is decently strong and the system will get more than loud enough to completely overcome road noise... with the windows and moonroof closed. With the windows open on the highway I can't really hear most of the bass, but I don't particularly care.

The steering wheel audio controls are awesome, though, and would be useless with an aftermarket head unit. Also, the dashboard panel in which the head unit sits contains the HVAC controls, so putting in an aftermarket unit will look wierd.