I love my M9 too. I've got what are technically "better quality" pistols in my collection, but it's still my favorite in every way. The only upgrades mine has had is a 150% recoil spring and some Hogue grips (feel wonderful). It shoots very accurately and quite hard too. It's the most powerful of all my pistols (the only one I can puncture a coke can with).
I have a 190% hammer spring for it too, but never had the balls to take the gun apart to install that yet. Waiting for the presence of a gun doc for that! lol
KJW make good guns. Their upgradability is limited, but the stock guns are great. Just beware of the mags, as they tend to develop leaks. I found that underfilling them (4-5 second propane fill instead of 7-8 seconds) helps preserve them.