I've done several mods to my Axe CP, more in the interest of learning how it works and building my knowledge for a future helicopter than anything else. The best "mod", as many have stated, is lipo batteries. They're lighter, smaller, and last much longer. If you really want to fly this thing, you have to have them. The second best mod was the addition of a GY240 gyro. The 3-in-1 board that comes with the Axe is very basic and will cause you to spend a lot of time correcting the tail. Once I had the gyro in, I went ahead and went all "separates" by installing an ESC for the main motor and removing the 3-in-1. I have the hs55 servos but I have not installed them.

You might want to consider the carbon tail boom--the aluminum piece will bend on a hard landing. douglas4's suggestion of a carbon arrow shaft is great. I bought the Helimax piece. You should also keep several spare parts on hand, notably rotor blades and blade grips.

Lastly, and probably most importantly, you can save yourself a lot of grief and expense by investing in the RealFlight simulator. It sounds like a lot of money, but piling into the ground in the sim time after time will make you feel much better about the purchase.