Portable jump starter

I was quite pleased to see that there exists such a thing as a portable jump starter, ie. a device that you plug in to the mains to charge it up, and you can then subsequently use it to jump start your car. This one at Maplin is an example.

Unfortunately, a bit of hunting around turns up reviews ranging fairly evenly from "it works, brilliant" to "it doesn't work, rubbish".

Anyone actually tried using one of these, and how does it compare with a normal trickle charger?