Horn tweeters aren't practical in cars. Instead look for great component 6.5" sets first. Then if you feel you need more highs, add extra hard dome/bullet tweeters. For component models and brands, read this forum. It's best you listen to one for yourself. Everyone has different taste.
forget horns. high end home audio uses ESL, or ribbon tweeters, or silk domes anyway. Horns are for PA systems and they suck.
look at focal, dynaudio, rainbow, or scanspeak and go with silk dome tweeters or DynAudio's Esotar tweeter. these will produce a very smooth, flat response. neodymium and metal tweeters tend to be brighter and harsher, which some people prefer.
Mostly what I was reacting to in the horn tweeters were the Klipch home audio speakers. I've heard some that sound fantastic. I'm not looking for blaring loud, but they gave a really soft tone and supported quite a bit of volume. If there is nothing out in similar quality for car audio, I'm cool with that. I've got pretty damn good Alpines in my jeep that work for what they are, but like I said, i'm looking for better sound quality.
There are better (arguably) sound quality drivers for car audio, that's what they were referring to above. Soft domes like the Dynaudio Esotars are used in $80,000 home audio sets, and are available for car audio use. Anyway, if you're dead-set on horns (many people prefer them, nothing wrong with that), Image Dynamics has component sets with different size and grades of horns. It's a tricky install and won't dial in really well without a 1/3 octave EQ, so be prepared for that if you plan to use them.
Not sure what to think about the Image Dynamics. Are they typically good stuff? I don't want to go horns just for the sake of it, and if I can get as good, or better sound, I'll probably just stick to what I know will fit without getting too crazy on the install.