They're great if you are playing without VATS. I highly recommend Blackhawk (the unique) if you are going to use them. They don't hold up very well, though.
One thing that bugs me about Magnums is, that I'll never get the most powerfull of them (Callahan's magnum), as I never destroy the BOS Citadel in Broken Steel.
The Blackhawk is pretty sweet. The other night I free-hand one shotted a charging super mutant master, not in sneak mode. As for the sniper rifles, I find it better to aim for the upper chest if I want a head shot.
Today, I've trained with the Sniper Rifle a bit. I've placed some bottles and tins on a table and tried to hit them.
My Sniper skill without VATS is getting better.
Scoped Magnums, I just about managed to keep one in high repair, they are not that plentiful, investing in wandering traders that visit Megaton may have made a slight difference, other trader locations occasionally had one, I tried different traders when I had stuff to sell and bought more ammo as well. (Sold quite a lot of Assault Rifle ammo along with the gun to Wastelanders and others, good to see them not-dead later).
I've collected every Magnum from the start, so I've many in reserve.
In my next playthrough I'll also have an eye on them.
Btw: I don't know if it's a bug, but the merchant Haley in Point Lookout can fully repair items.
In my first playthrough 2 traders died, too bad as they are a good resource for additional items and weapons. In my second I reverse-pickpocket Hellfire armor and Plasmaguns. That seem to keep them alive.