When I picked up my Slash the LHS sold me a set of DTX 4200 6 cell. I get plenty of power (for me since I'm new) but I only get 20 minutes or so of run time. Doesn't seem like very long. Maybe its the brand since they were cheap.

How many extra batteries should I have? And do I really want to swap one of after another? Or do I need a better set that may last longer?