I have owned and used the same KJW M9 elite for close to 5 or 6 years now. It gets heavy frequent use during my playing season and has only suffered one minor failure but that was magazine related.
All internal springs are replaced every year or two depending on use and I just recently added new reinforced loading nozzle, not that the old one was broken but it was getting worn down and this was more of a preventative maintenance act. The magazine catch on the M9 seems to suffer from heavy wear mainly because the older models used plastic catches. The magazine catch has been replaced in this pistol 3 times now. The first two were plastic catches while the 3rd existing catch is all metal and currently shows no signs of wear. The factory grips have been replaced with real hogue grips which I find are much more comfortable than the standard plastic grips.
Depending on temperature I can usually average 2 real cap magazines and sometimes even three before I need to regass. Although I prefer to top off whenever I can.
With good maintenance I hope to keep this GBB running for a few more years to come till I retire it.