I've never tried it, but there are a few issues that spring to mind.
1. Sanitise thoroughly and you should be OK from the infection front.
2. Plastic is not a complete barrier from oxygen, so you will get some staling if you leave your beer in there too long. How long is too long is anyone's guess, but I'd be wary about anything longer than a month or two.
3. Fresh wort containers would be ideal as they're free. The one I've got is 17L, so you can fill it right up and avoid the oxygen in the headspace problem. I've used it for cold conditioning an ale and it worked well.
4. A tablespoon of dex in the 20L jerries would probably work pretty well too.
5. Why did I number all of these?
