David,
I have the Power Tank, 7 amp model which cost $60. I have flashlights that I've paid more for. I use the Power Tank with my Celestron Nexstar 6 SE, sometimes for three or four hours. When I get home and plug it in to recharge, it's ready to go again in less than an hour. I can also use the Power Tank with my Meade 10" ACF. I think the Power Tank is probably the best investment dollar for dollar of any astronomical gear I've ever bought. By all means, get one! They're not expensive and they are a lot quicker than putting in 8 batteries!