IME these devices all depend on the quality of the battery used. It's a physically small battery to deliver enough current to start a car engine so the battery inside needs to be top quality.
Quite often they use cheap batteries that aren't up to the job, my 20 quid maplins one (which looks identical to the Machine Mart one!) being a case in point. I bought it for powering camping lights for which it is excellent, but it didn't come close to cranking a 2l 4 pot..
Then there's looking after it. They generally come with cheap chargers which don't keep the battery in good condition and can overcharge it if left on trickle charge for long periods. Constant charging without a regular discharge won't help either.
My feelings are that these are great for providing 12v power from a portable unit, but if you want something to reliably start a car, you are best served by either jump leads if out and about, or a good quality mains charger if mains power is available.
If the battery is not a lost cause, a mains charger will get you up and running again in 15 or 20 minutes and jump leads will do it immediately a battery pack will be a lottery, IMHO.
Having said this, I once got stranded with a knackered battery and borrowed a professional jump start battery pack from a garage round the corner. They were not sure if it was charged and I didn't hold out a lot of hope. It started the car without an issue at all. Can't remember what brand it was - maybe Snap-On or something. It was a totally no-frills battery in a box - no tyre pump, light, etc.
Kevin