I have 3 sets of infinities in my car. I have 6x8 Infinity Reference components in the front doors, Kappas in the rear doors and 6x9 Kappas on the rear deck.
The components are really good, the sound is steady and the highs are clean. I don't hear much of the Kappas in the rear doors, but they are still good, although the component references are better. The rear 6x9s are great, full bass even when I turn off the sub, the highs are good too.
I have Alpine V12 55rms x 4 for the door speakers and Alpine V12 90rms x 2 for the rear 6x9s.
Overall, the infinity makes great speakers, I have Alpine type-Rs in my 325i, they are good too, although no amp there.
I have Sony 6x9s in my Mercury and I can say that after 5 years of usage, they SUCK! Although sounded OK when new.