I bought one so that I could power my macbook pro from the 12v supply in the house I'm building. Tested it and all works as advertised. cheaper and more efficient than an inverter.
I used it to charge my laptop from the bike when I was on a two month tour of the USA last year. When I stopped at powered campsites I could then power the laptop from a plug socket. Also used it to charge my phone, ipod etc.