In my semi-long and dismal life I've owned just about every gun that anyone with taste could possibly want--except a double rifle. There is no earthly reason for anyone to own a double rifle, just as there is no earthly reason to own a ZO6 Corvette, but if you gave me either I would not turn it away.
I came very close to owning a double rifle in the early 1980s. Safari Outfitters, which was then in Ridgefield, CT, got hold of a Westley- Richards Droplock, with barrels in .300 H&H, .375 H&H, and .458. It had a American-style stock FOR A LEFT-HANDER, and had been made in the 1960s for a majarajah who had never used it. The rifle cost $30,000 and I seriously considered taking out a second mortgage to buy it, but I didn't.
Not only have I never owned one, but I've never hunted with a PH who used a double, or owned one. Mostly, they cost too much, and they are useful only on dangerous game. There is a myth that double rifles handle like shotguns, but that is a crock. A side-by-side shotgun weighs maybe 7 pounds while a double rifle in a serious caliber weighs anywhere from 12 to 15, and most of that is in the barrels. You tell me how something like that is going to handle like a shotgun.
But a double will give you two very quick shots, and because it's more compact than a bolt gun, it can be very quick to maneuver in thick brush where much of the fun takes place.
If you're in the market for a double, here's some advice: Don't get one in .375 H&H or smaller. A true double is .45 and bigger. Probably the most popular cartridge is the .470 Nitro Express, and if you can take the recoil, the .500 Nitro Express is even better. Get a boxlock rather than a sidelock; the latter cost a fortune if they're any good.
Right now, I think the two best using doubles on the market are the ones built by Butch Searcy and Blaser. Searcy's rifles start at $15,000, and the Blasers begin at around $10,000. Both are first-rate working guns. The Blaser in particular has the best iron sights for a dangerous game rifle that I've ever seen, and you can carry it loaded but completely safe, which may prevent you blowing someone's head off besides that of the buffalo.