I've yet to hear a factory head unit that holds a candle to the aftermarket stereo setups I've had. The tweakability alone is enough I think to be able to tune it to sound better, even if the overall product quality wasn't that much better.
But that's all personal opinion, I've gotten into several internet debates with dudes who claim their factory system would kick my aftermarket system's ass, and since we're in different cities and I wouldn't agree to meet with a nutjob like that anyway, I'm never going to prove it. Your mileage may vary. I ride in enough newer cars to form the opinion that they really cheap out even on their premium stereos.
I'd suggest you buy from someone with a satisfaction guarantee like crutchfield. If you buy a head unit and don't think it sounds better, they'll probably take it back.