I'm shocked that you say you don't see people run them. Sure seems to me like EVERYONE has one. My experience with aluminum driveshafts has been generally positive. It
will not free up 10hp though.
I had a 94 GT that had such a bad vibration on the interstate when I got it that it would pop out of fifth gear. The bellhousing was cracked on it, too. Replaced the bellhousing so I could drive it and still had the vibration popping me out of gear. Figured I'd replace the driveshaft the following weekend. Then, when I went to do it, I found that my trans was cracked where it had been fine before! Replaced the transmission and BH AGAIN and put the aluminum driveshaft on. Vibration gone. Of course, that was a little different scenario than most. My steel one was likely damaged or out of balance based on how bad mine was.
Back when I had a former dirt track 302 in my current 95 Mustang, I dropped the clutch on the drag strip and revved the heck out of it, doing 6000 RPMs in a standing burnout. I've started in second and ran it to the rev limiter doing burnouts. No problems and I'm still running it today in autocross and track days and very occasional street driving. Now I have a decently strong 351w in the car, on 200tw decently sticky tires. I leave the line as hard as I can in AutoCross, and it's held up just fine. Car's been to 130 and it felt smooth as hell.
It's one of those things that doesn't really hurt you in any way, but doesn't really get noticed too much as a
help either. However, if you're doing a form of racing where rapid changes in engine RPM are required, it may be more of a help. AutoCross, road racing/HPDE, etc. You could combine that modification with a lighter flywheel and wheels to do even better. Of course, that's when you're trying to get all the power wrung out of a given combination...there's much more impactful changes to be made.
@lwarrior1016 versus the stock steel one? I'd pick the FRPP aluminum one between those two, personally. The aluminum driveshaft is stronger than the stock one and better balanced. Virtually no downside on a street car or mild build. That's the reason that Ford chose aluminum for the Cobras. If you're serious about the drag strip, a higher rated aluminum one might be smart. I know DSS makes them rated for like 950hp.