I don't use the Marathon Plus but I think mine are similar which are the Marathon GreenGuard.
I use those on my touring bike and I got to say...I like them. The odd thing is they are heavy tires of course, but they roll really well and I can't even tell I have heavy tires on, nor do they feel like wood tires, in fact with touring weight on they feel quite nice, now if I ride the bike without any extra weight I reduce the tire pressure to 45 front and 70 rear, but when I add my gear of 45 pounds approximately (I weigh 175 with clothes and the bike weighs 25) I raise the pressure to 60 front and 85 rear.
I think people who complain about the ride quality simply have too much air in the tires, if you use this tire pressure calculator, and use the second one not the first):
http://www.dorkypantsr.us/bike-tire-pressure-calculator.html you'll get real close to your idea pressure that will make the bike ride better and make the tires last longer. But again I do use the Greenguard ones so there might be some difference there that I'm unaware of in the ride quality and traction.
I also haven't noticed any grip issues with mine, even in the rain they performed excellently, I don't go touring in the winter so I have no idea how good the grip will be in the winter, but I would suspect that since the tire's tread is made to last a long time, which means the tread is thick and hard, it will not be very good in really cold weather because the rubber will get and lose its pliability. Granted they're not made for racing with, so to expect high speed is a bit lame, that's like saying you bought a Toyota Prius so you could get 50 mpg but are not happy with it because it won't do over 200 mph! You buy the Marathons for touring, durability, and puncture resistance that is second to none.
They are difficult to put on but I was able to do it without resorting to the straps that the video shows but it is a interesting method, all I did was set the tires in a 150 degree oven for about 20 minutes, my wife was none to happy about that but when it was all done there was no burnt rubber going on inside the oven so she mellowed out! I took a tire out and installed like I would normally which is very similar to the video. I used a set of Soma steel core tire levers which are the best levers on the market to get on the last 5 inches, and toward the last 2 inches I had to resort to the VAR tire lever which snapped that last section in. After I got both on my hands were quite sore.
These tires do not retain air any differently than any other tire, the tube holds the air pressure the tire doesn't hold any pressure because it's not a sealed system like tubeless tires, thus the tires will slowly have escaping air pressure like any other tire at the same rate. I haven't noticed any difference whatsoever with psi loss.
Experience wise I've had horrible luck with anything made by Conti in the road tire area, but the MTB tires seem to be just fine. Gatorskins don't hold a match to a tire like the Marathon Greenguard. You can tell by feeling the tire that the Greenguard is a heck of lot more robust than a Gatorskin; the Greenguard is a truck tire designed for heavy loads going long distances, the Gatorskin is a compromise between a racing tire and puncture resistant tire like the Greenguard and the Gator doesn't excel at either. I've used the Gators and I wasn't impressed, especially after getting 7 flats before something damaged it's fragile sidewall ending the tire's career short. The older version of the Specialized Armadillo All Condition tire at the time had the toughest sidewalls in the industry, I actually rode one flat just to see what would happen to the sidewall, the sidewall was so strong it even protected the rim from damage, I rode it that way for 5 miles back to the house; yes some of you are thinking why didn't I just fix the tire, because the reason it got a flat was only because I had worn the tire down to the threads and beyond just to test it to find out how tough it was. Well the Marathon sidewalls are even tougher! I still have an old Armadillo and I compared them, and the sidewall is bit thicker and stiffer than an Armadillo, and the tread on the Greenguard is thicker.
Even though I commute to work I don't use the Greenguard for commuting on, that is only my touring tire, I currently use Specialized Roubaix Pro tires they haven't flatted yet but I've had a lot of tires last without flatting while commuting.
