As for Impact Drivers. Yes, they do prevent slippage and are AMAZING for driving screws.
regarding the driver I got from HF... the battery only lasted for 15 screws before the battery started to run down. that was pathetic and frustrating.
I now know I really, really want an impact driver so I'm going to return the HF and pick up a name brand from Home Depot today so I can finish my project with hopefully 2 charges or less.