I have one and it is a great rifle.  I previously purchased CA's M15A4 and was so pleased that I got their 36C.  When I first got it I had to readjust the hop up as it was not shooting straight.  I have also put a M12 spring in it.  Great gun.

Here I am, a newbie and already giving advise.  If any of the "Old School" players want to correct me, please do.  

I purchased a CA M15A4 as my first gun.  Great Gun!!! You made a good choice.  Only, do not upgrade the gun yet!!!  I'll let you learn from my experience.  When I first got the gun, the FPS was on the low side so I got a M120 spring to try and bring the FPS up.  WRONG!!  Once I cracked open the mechbox, and replaced the spring, the gears would not pull the piston back and if it did it only went so far then jammed.  After many, many, many hours of trying to get the shimming right and then going to a M100 spring, my gun shoots like a dream!!!!   Also replaced the cylinder, piston to bring up the FPS as my stock parts had NO seal around the piston head.  Also went to a small bore barrell.  

Now, if I were you, I'd not mess with the gear box first, get a small bore barrell, will improve effeciency a little and most of all accuracy.  Then after you have played with the gun for awhile, get a stronger spring.  

Good Luck