It seems to me that handloading has allowed me to play with cartridges that never succeeded but were still rather unique. My .445 Supermags are simply stretch versions of the .44 magnum and not in the same league as the .454 Casull.
Aubrey White's Imperial line is more than 10 years old, but it spawned the RUM line of cartridges.
The .338 RUM is more efficent than the Lapua and virtually identical ballistically. Nonetheless, I opted for the Lapua due to availability of cartridges worldwide (although it's still hard to find).
The .308 Marlin is doomed. So is the .30 TC and the .450 Marlin. The .376 Steyr is fun, quite efficient, and available in a handsome carbine, but it is doomed.
The .405 Winchester is a revived antique and respectible, but doomed. Sorry about that, I know other's hope it will last.
I disagree on the .375 RUM. It is efficient and offers somewhat more velocity for a flatter trajectory than the H&H, but it doesn't create the hideous blast of the .378 Weatherby. It's also available in a neat carbine with Hogue overmolded stock and according to friends, is selling like hotcakes in Alaska.
I've been wrong before, most notably with the .357 Sig which was adopted by several police agencies.
The .45 GAP is a thorough flop. So was the Winchester 9x23, (not the old largo). Both are dead, dead, dead.
The .260 Remington didn't exactly do anything for me and a lot of others seem to feel the same way. It's going to be around but will eventually die off.
The 6.8 the yanks are fiddling with isn't exactly doing much because it really doesn't do that much. The 6.5 Grendal is far superior, but seems to be going nowhere.
The entire line of Lazzeroni cartridges are terrific if you don't mind fitting a new barrel every thousand rounds. Sako chambered their TRG-S for one or two Lazzeroni cartridges for increased appeal due to reduced cost, but they didn't sell. They are doomed as likely so shall their company be.
The entire line of Dakota cartridges is about gone due to the collapse of the company. It has been sold and reorganized, but no one else chambers their cartridges. It didn't help that they put so much capitol into cartridge development and marketing. Darn fine guns, though.