I have a pair in my 67 fastback. They came with the car when I bought it, but the po never painted them to match the interior. I painted them with interior paint from NPD. They look fine now.
I think that the speakers are from alpine on the set that I have. I think they sound fine. I just don't think that they go loud or deep enough for me. It doesn't help that the sound is coming from your feet! I am buying a 'sound bar' from custom autosound so I can fade the sound with the sound bar rear speakers.
Overall, I think they are much better than a single dash speaker. But, I would plan on running rear speakers also if you want any sound quality.
I bought the preformed kickpanels and added my own speakers. I ended up having to add a spacer to get the speakers to fit.
I don't know how thoe kickpanel speakers with the speakers already installed work out. Maybe they use a shallower speaker.
Anyway the sound is good. Doesn't quite keep up with my 6x9's on my rear shelf but that's to be expected. There's no rattling or anything and I enjoy the sound quality. You can sort of see it on my website (page2).
I initially bought them with speakers. The speakers they supplied were pretty poor so I returned them for the panels without. I was able installed a set of Sony 6" Xplod speakers (about 1-3/4" deep) that sound pretty good. You'll still need something in the rear of the car, but it beats the speaker in the dash or cutting up your doors. Like Kent identified, you will lose the space between the clutch pedal and the kick panel if you have a manual transmision, but I found it's something you can live with. Good Luck.
I have them with Sony Xplod's as well, along with 3 6x9 Xplod's in the rear, a 400 watt Xplod amp and a 10" Xplod subwoofer.. The sound quality is excellent and the only rattle I've found is the license plate... Typical for most cars with heavy bass.
My speakers are on order. The problem i had with selection was a lack of depth for the speaker (< 2 inches). I chose component speakers to get the most sound out of a little space. In the back i'm matching them up with a set of 6x9's. All with 75 watts rms rating and a 12" sub at 180 watts.
The mustang seems to fight having a quality sound system.
I have to chime in and say that I too bought a set w/o speakers and added XPlod 6s. I also run XPlod 6x9s out in back, and the combo is pretty good, although by no means an audiophiles dream. Painting them with '66 dark red interior paint from a rattle can worked out surprisingly well, and matches the other interior parts.
I think the hardest part is getting the kickpanel fastened in really well so that the speaker doesn't rattle.
I also have a C4 tranny, so the protrusion into the leg well doesn't bother me.