I have Prime because my parents could add my account to theirs for free. I only get the free shipping, not all the media, but it's great. My account is completely separate to theirs, so if anyone's thinking about it couldn't you team up with a friend or family member to share the cost?
Re: berries: I have my grocery list for things I want to buy that time, but I also have a standing grocery list for things it's always OK to buy. Almost all of it is cupboard items that will keep (pasta, noodles, peanut butter, tinned beans) but there are a few fresh things on it. Apples are the one that spring to mind - they just WILL get eaten, so if I see they're cheap I'll pick up a few bags, secure in the knowledge that they won't go to waste. So... I think it's important to distinguish between impulse purchases and unplanned but still legit purchases.