I've put over 15,000 flat-free miles on my Marathon Plus tires with Mr. Tuffy liners. The ride is smooth and they work fine in the wet. I came to cycling with automotive expectations regarding bike tires, and the Marathon Plus are the first tires I found that met my minimum standard of matching car tire durability. I've had a flat on my car during this period but none on my bike. The bike and car are driven over similar annual milages.
Slime: No increase in puncture resistance, but makes an amazing mess.
Armadillos: My wife's road bike has them. The ride is unpleasant and they do not have the flat resistance of the Marathons.
Mr. Tuffy: Greatly increases flat resistance. I always use them.
Paul