I've gone through two of these in the last two years: one I bought myself and one bought for me as a gift. They are cheap, nasty, and do not perform.
As others have described, the unit will not hold a charge and cannot be relied upon to start your car, let alone carry out the far lesser task of putting air into tyres. Like a punctured spare, it's something you grow to rely on yet will leave you stranded at the crucial moment. You're actually better off without it.
If you spend that little bit extra on the Draper equivalent you'll find you have something that should last you five years rather than one, meaning it's actually the least expensive long term solution.
As we're beginning to discover, cheap is so very rarely cheerful. Quality is remembered long after the price is forgotten!