They work great. I've used one any number of times. Some will also charge cell phones or other devices. Some Antigravity jump starters also have a voltage booster that can be used to charge a laptop.
Go oversized. Sometimes an engine can be hard to start and you need extra power for another crank or 3, especially in cold weather.
That said, I'm guessing you're either using the cheapest batteries made or you have a power drain somewhere. Pull off 1 of the battery cables and put a multimeter or test light between the cable and the battery to see if it's registering voltage when everything is turned off. A battery in my Polaris RZR lasts 4 to 5 years.
