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".
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?