I own a Blaser S2 with barrels in 500/416, .375 H&H, and 30-06. I have used it on everything from cape buffalo to duiker with the .375 and 30-06 barrels. I have killed game from fifty yards to a bit over two-hundred with it. I should note that I also own traditional doubles in .470 and 9.3x74r (x3).
The S2 is different than most other double rifles. It is extraordinarily accurate, has perfect ergonomics, excellent open sights, and is designed from jump street for a detachable scope or other optic sights. Sadly, the fact that it was not traditional garnered it a lot of negative press. Terry Weiland, whose writing I otherwise like, absolutely hated it, and the majority of gun scribes jumped on that band wagon. It simply wasn't a traditional double. If you search diligently here, you will find a couple of posters attempting to criticize my experiences with the rifle. I apparently wasn't using a double "properly." Proper or not, I was certainly pleased with it taking a wonderful free range Mozambique nyala at 175 yards.
The "automatic safety" observation is one of the most common criticisms of the rifle. For me this is a non issue, but for others it could be one. I primarily hunt with SxS and OU shotguns. Probably 3/4's of those made after WWII have auto safeties. When either hits my cheek and shoulder the safety is automatically being pressed forward. I do the same with the cocking lever of the S2 (or one of my Blaser R8's). If I want a silent reload, a slight down pressure on the cocking slide releases it and the barrels drop open silently.
The tilting breach block also gets criticism from people who have never used the rifle. Reload is a non issue, and unlike any traditional lock, the S2 will never shoot loose (it is impossible for it to go off face). From a price perspective, a lightly used one will be in the same general range as a Merkel. I think it is from a design, accuracy, and ergonomic perspective, a far better rifle. But, it is not traditional.
If your intent is to plug a pair of holes in the shoulder of a cape buffalo at 60 yards while wearing your Stuart Granger hat (a worthy aspiration), then by all means get the Merkel. If on the other hand, you want to be able to take the kudu of a lifetime on the nest ridge while trekking the spoor of a buffalo, the S2 is hard to beat.