I have them. I would consider some of them slightly, just slightly, better than the original weapons.
The Perferator and Combat Shotgun are really devestating.
The Cowboy Shotgun does require some time to get used to. It's really powerful, but since it's a double barrel you'll need to reload quite often and you can not have that many shells on back up.
The Machine Pistol and the Combat SMG are really useful, espeically the Combat SMG.
The Matilda is so far the only weapon that I wouldn't consider better than the original weapons since it's extremely weak and it's not good at medium and long ranges. But if you're really good then you can get used to the gun really quick.
The Anti-Matrial can either be good or bad, determining what kind of sniper rifle you perfer. If you perfer a Semi-Automatic then the Anti Matrial gun is going to disappoint you since it's a Bolt Action (Which doesn't make any sense because a Barret .50 Cal is a semi-automatic), but if you can handle a Bolt Action then this weapon is going to be really useful.
I haven't tried the Classic Launcher, but if it's like the regular grenade launcher that can be found scattered around the levels, then the Classic Launcher is really useful.
These weapons are really good and useful sometimes.