I have had mine for a few years. I don't shoot it enough. I purchased the Marine Magnum because I wanted a shotgun I could have around without worry of degradation from long term storage. However, since I break it out to shoot 2-4 times a year, maybe shooting 100 rounds each time, it doesn't store long enough for their to be any sort of degredation. This is especially true since I am not on a sailing ship or in some other harsh environment. So I could have saved some $ and gone with a blued or parkerized version.
With that said, I also liked the gun because it would handle 3" shells and had the extended mag on already. While I like to keep the option of the 3" shells, I don't enjoy shooting them too much. The extended mag is great and has been no problem at all.
As for the shiny finish, as with other guns, you can home custom camo the gun in Duracoat or other paint.
I would not say I am unhappy with the purchase. I did not need the nickel plating and I paid extra to have a gun with it. And with that said, I am very happy with the gun in terms of function and durability, but those are hallmarks of the 870 platform.