Looks interesting but I agree, you can buy a spare Mac battery even from Apple at under half the price an get the same increase in life. Only advantage with Hyper Mac is you don't need to turn the Mac off to change the battery.
The latest Macs portables may present a problem as the batteries are almost part of the Mac itself. However you do get exceedingly long life with them so you may not need the additional battery. The Car Charger looks good though very expensive.