Super Tweeters

There are several manufacturers of add-on supertweeters to extend the speaker response well up into supersonic regions. These are reported to "enhance" the sound, particularly of SACD reproduction. I've seen reviews in the UK hi-fi press. The units I've read about include the Sony, Tannoy and Townshend units but I'm sure there are others. What are members experiences with these devices?
Custodian I asked the same question a few weeks back-do a search on Super Tweeters in the forums you'll find some good information.
I have experience with speakers that have integral supertweeters. There is an audible difference with some "dulling" of the high frequencies when you cover them.
Custodian, you may want to check out the ion tweeters from Acapella. You can purchase them for use with other speakers. I believe the Freq. responce is 5Khz-40Khz. They are the most open and real sounding tweeters made.
I have the ST-50 made by Tannoy and couldn't live without them. Depending on the frequency setting you choose, they can add a little sparkle to the highest treble, but no matter what cut-off point is chosen, the whole presentation becomes more liquid and musical with much bigger soundstage. Strangly enough, things also sound "faster".