The problem with the standard tweeters is the same as most car door tweeters, it's the crossover that makes them sound awful. Especially when tweeters are generally more efficient than the main speakers.The HF unit itself is fine and I wouldn't replace the stock ones in my Spider.
The "crossover" is usually a series 4.7mf capacitor which basically just keeps the LF signals from damaging the tweeter. It has a very shallow ("first order") roll-off of the frequencies below the crossover freqency which means that you get a huge amount of mid-range squawk from the tweeter, which because of it's position masks the main speaker and when you're driving all you can hear is this mid-high range signal. Not nice.
I have an audio spectrum analyser, and used it to tune the stock tweeters in my Spider. I now have a third order crossover using two capacitors, and inductor and a resistor to lower the sensitivity so it matches the main speakers.
The aim here is to allow the tweeters to just fill in the very top frequencies that get lost when the main speakers are pointing at your legs. They should integrate with the main source, not take over.
My Vauxhall Astra tweeters were 20dB louder than the main speakers and de-waxed my ears when loud, they now have a 12ohm resistor in series with a third order crossover and now you wouldn't know they were there except I can hear every detail in music.