Well it depends which onboard sound you are talking about. I was quite impressed by the ADI2000B on the P5Q Deluxe...seriously the best onboard I've ever heard. They even drove my HD 580s decently.
It was not as good as my modded xfi but it certainly was better than anything a few bucks would get you.
I remember when I had my P5K the onboard was terrible, and my P5B Deluxe was even worse! I remember when I had my Q6600 on it it would screech and stuff during gaming lol.