i've got one, using it to power my sony vaio pcg-fxa > usbpre 1.5 and it works very well...it will keep it going for 3+ hours on a charge (and this is a big laptop), and then i've got internal batts i can hot swap when necessary to keep it running for the rest of the show. YMMV with a different laptop. the price is right also, i think they're like $300. i tried another laptop batt that didn't perform as well and was about $60 more from ebatts.com.
what i really wanted to try was the electrovaya system, but they weren't compatible with my laptop.