Barrel, upgrade board/firmware depending on your needs.
Speed, Consistency, reliability
This was my first high end marker. It shot practically right out of the box (I needed to add an asa). It never let me down. All I ever changed on it was the barrel and the trigger. The stock trigger is perfectly fine, just not my style. I changed to one that fit me better.
I like the trigger guard, it is larger then most. For those out there that have larger hands/fingers this is definitely the gun for you.
If you take care of this gun, it will take care of you for a long time.
This is a good marker. If you can get it for the right price, I would pick it up.