what are the thoughts of stand alone super tweeter

i want to buy a pair of totally stand alone super tweeters, with all necessary parts built into the super tweeter, just place it on top of my speakers and run speaker cables to piggyback my present speakers connections or even come off my power amp.please tell me pros and cons. price seems to be from $500-$3000 except for the radio shack built years ago.which sell for about $80.00 used........... audiogon has the high end ones all the time, is it worth my money... regards forevermusic414