Yeah I've seen those! They look cool but I would never trust it on a big stage or on tour. Once you've set up, gone through soundcheck, accidentally left it on and player a show I think I would be nearing the end of batteries charge.
I actually had one of the old rockboard ones and it just went mid gig, loud bang through the PA, fully charged and the dead as a door nail.
I should really buying a proper £200 power supply but just very skint at the moment!