For a super tweeter to work it needs to cover a range from 20kHz and up. This can add spacial cues that help open up the sound stage and imaging. And it is not hard to DIY one. You can accomplish a similar effect from an up firing or rear firing tweeter.
However, the majority of the time they are improperly implemented by allowing them to play down into lower ranges also covered by another tweeter or another driver. This will cause cancellation effects and holes in the response. So don't always look at them as being a positive.