Do they work? How are they for muscle building? Fat loss? How can I make a plan based around them? My gym is always empty and has a decent amount of equiptment so I want to try them out.
They are just just a way to organize your sets... Nothing special about them...
All the "science" saying that they allow for more weight being used or faster recovery or whatever is off-set by you putting more strain on, for example, your forearms (when super-setting upper body work to cut rest times short). That can give you some trouble (tight forearm musculature/tendon inflammation when overdone, weaker grip)... And actually fatiguing your back is not necessarily a great idea when you are benching heavy. You may end up being unable to keep a tight set-up then, and that may cause all sorts of issues.
So, gain something here, lose something there.
IMO if you like them/they make things more practical for you, use them... But expecting some kind of noticeable effect on the speed of your gains or some such? Nah.
Fat loss on the other hand... If you actually use your training as a means to lose fat, then perhaps... But then Barbell Complexes or some such (or just forget about that crap and do HIIT if you're into higher intensity fat loss work) are better suited for that...
I'd stick with diet and cardio for the purpose of losing fat, and leave strength training as a means to keep/gain muscle and strength depending on the situation.
Only times I've found them somewhat useful was when training smaller muscle groups while pressed for time. I don't like using them on bigger exercises... Sometimes I'll do some face pulls or some such between bench warm-ups and so on, but I would not call that super-setting...
I would also not consider them a factor to base your routine around. Just do what is practical in your situation, you're not going to lose out on any gains just because you aren't super-setting everything. Put them in where they fit.
Easy answer is no.
Try them out for yourself and tell us what you think. It might be magic.