The main reason I now do all my own 'tuning' (well alright just fettling and tidying up really) is that I once paid someone to sort my Lightning out for me. At that point I had never opened up a spring rifle.
Got it back and after a week or two still wasn't happy. Followed some online instructions and opened it up, and it was pretty clear that it was the first time the gun had been disassembled. Meaning that this person had taken my money for nothing.
For similar reasons I won't send kit off to be modified if I can help it, after Nick from Magic 9 disappeared off the face of the earth with my 586 sight assembly.