Are any of the unique/"purple" weapons worth it by the time you run into them (unless you rely on metagame knowledge to beeline there, avoiding enemies, early in the game, I guess)?
The unique fist weapons all have some pretty nice things going on. Unique shotguns are good too (except the joke one, but it's obviously meant to be bad). The unique acid pistol is great too, though it does have its drawback. If you can work around that it's vastly superior to normal acid pistols.
The rest aren't that exciting. MP5 is nice considering how early you get it though. Dragunov can be too.
These weapons used to have bugged properties that made them more useful and unique, like no STR requirement on the Dragunov, or no penalties for moving, but that's all gone now.
Well, the Dragunov still seems to be bugged, just it's a bad bug. It doesn't have a scope. Every other rifle gets bonuses from a scope, but it doesn't.