I'm around 7,500 kills now and really starting to get into this game.
Rather than just go in guns blazing I'm starting to play much more tactically and it is paying off. I can tell how much I have improved as a player. I've also finally dragged my kdr from the gutter to above 1.0 so that speaks volumes after over 7000 kills. A bit sad having kdr in the back of your head while playing but it does help to balance your offence and defence.
I realise this is common sense for veteran FPS players but this is the most I have ever gotten into an FPS.