I used them all the time on FO3, but not so much in NV, as most ammo that you could use it for MF Cell, E cell are not found in containers unlike in FO3.
Guns however you can just use the reloading bench and buy loads of cheap 22lr, 9mm, 10mm, 375 mag, 44 mag, 20 guage ammo for 1 cap and make whatever you wish.
I see it coming in handy some times but as the game is very linearally spaced by the time you reach vegas you should have at least one gun and ammo for free that can perform at the specs for those areas.
That and merchants and enough caps to provide the best ones with ammo, ( * if you follow that path exactly, and not risk high level critters early on ) .
The test of the perk is head to Nellis airbase and see how many missiles you find in containers, on a normal playthrough it's roughly 20, if you double this then thats quite a few caps.