I think I once got amazon prime through amazon mom for 3 months (or maybe even 6). I also got prime for 1 month on two other separate trial promotions, the normal one and another one that was eligible with the purchase of a qualifying book - IIRC, the ability to sign up for the trial came up during the checkout process, allowing you to get it without actually completing the checkout.
I'm in Canada and Prime is absolutely useless for shipping here, but I regularly use a guy in CT to forward me purchases (for a small fee) that get sent to his UPS mailbox if the item(s) is/are impossible to get in Canada, or just so much cheaper (even with the fee) than buying from a Canadian retailer, or an American one that will ship the thing directly here.
So I wouldn't come even close to getting my money's worth by paying for Amazon Prime, but the various trials have allowed me to get it cheaper and/or faster than I otherwise would, and it's even been useful enough that I created 1 or 2 other accounts at some point in order to be able to sign up for the trial again. Which I'm aware is wrong, but they've made such a small fortune off me between the US and Canadian sites that I don't feel too bad about it, even though I'd never do that sort of thing with a smaller company or one I only OCCASIONALLY buy from.