I have the 300A pack, and it's served me well... Remember to plug it in overnight after use or after a month and you'll be fine. 500A might be overkill unless you have a big truck/SUV to do, and do you really need the halogen light or compass?
I got it on sale for 39.99 so right now at 35.99 it is super cheap.
http://www.canadiantire.ca/AST/brows....jsp?locale=en
We used it to jump our minivan whose battery was having problems, and my friend's BMW because he left his lights on overnight. Worked great quite a few times.