are they really worth it or a waste of money?
On a decent modified engine with a high output alternator, yes. Otherwise, not really. as the HP gain would be nominal.
so more looks than horsepower
Not like many people see them.
I've used a set the whole time the 5.0 been in the TC.. Yes you'll get a small incease in power(maybe 5hp, this is from my experience with a stock 5.0 Stang). You do have to keep a eye on the temps, as mine doesn't like idling in hot weather. Also a 3G alt swap is a good idea...
I've got underdrives on my 87. I never had any cooling problems (I run a 195 t-stat). Even in stop and go traffic on a 98 degree TX summer day. I do have a 3 row copper/brass radiator though. I haven't had charging problems with the stock alternator either although at a stop light the lights get a little dim. No problems with the stereo but I dont have an amp on it either.
I would say at least upgrade your radiator if you haven't already and if you have an amp go with the 3g alternator.