Here is a link to a quick writeup of my Polk db650 and db525 speaker installation that I completed today: speaker installation. Warning: total picture download is ~5MB.
The speakers sound good driven by my DEH-P550MP head unit. Crutchfield and Circuity City.com both have a sale right now for buy one pair, get a second pair 1/2-off. These speakers are so clear. I heard detail in some songs that I have never heard before. Bass is punchy, but not deep. I definitely have to turn my head unit up louder to give these Polks the power they want. I'm sure an external amp would make them sound even better!