I had a KJW full metal around 5 or 6 years ago (it was the two tone M92FS Elite, but really the same thing). Worked great and never had issues with it over the several months I owned it and I estimate I put around 1000 bb's through it. Just keep the mags well oiled and charged during storage and you will not have a problem.
As for the pistol, after every use I suggest you clean and oil it properly then store it in a clean dry place. As stated before the quality of the metal is alittle low and its most likely a high concentration of magnesium so it is prone to oxidation. Dutiful cleaning and oiling will prevent it from doing so.
Personally though, for the cost I would rather pick up a WE (Im taken by both their 3.8 Hicapa and 1911SV). Frankly they seem to be better built and are around the same price point. Although I hear the same complaints about their mags. Though they are alittle harder to come across, as I know of only one retailer who is offering them.