BacPac should be pretty awesome since just for the fact it gives you a decent menu structure alone. That plus video preview seems like it's well worth it. I hope so, cause I ordered one myself and it should be here in a couple of hours :] Will let you know what it's like.
I have the LCD backpack. It helps a LOT, but it obviously sucks up the battery, especially in the cold snowboarding environment. It's better for when you're using the GP as a still camera, or looking at your vids while you break for lunch.
As for the mount...it comes with a bunch of them. Should be two of each kind (except the harnesses). I put one on my helmet...but then bought a new helmet. That old mount is history. You're never getting that glue off. The helmet straps works pretty good (I've used it whitewater rafting, since I was using their helmet), but I just don't trust it without a couple of twist ties through the vent holes, or some other way to secure should you take a hard fall or something. I've never had it fall off -- it's got really secure rubber grips, but still, it's be an expensive loss if it happened.
The best tip I can offer is to really memorize all the damned click sequences. Nothing sucks worse than finding out you missed one click and wound up recording all your lift rides up, then turning the camera off for your run down. Yes, I've done it. Yes, it's easy...the beep isn't nearly loud enough, especially when you're wearing a helmet and a lift is humming nearby and a snow gun is blowing, etc, etc. If you're with a buddy, ask him to check to make sure the red light is blinking before you take off! Or, wear it on your wrist or chest or a place where you can see the red light. You'll get some neat perspectives, too. The helmet-top perspective gets boring after a while.