we give them out as gifts at my co. they are accepted anywhere you see a visa/mastercard/amex sign. we buy them online from wells fargo, but you can also purchase them in stores. you normally pay a fee for the card and charge it up. the visa ones we get have a limit of $500, but I know amex go up higher than that.
you can check the card's balance online, or by calling a phone number in the back.
edit: crap, I could have sworn you were asking about visa giftcards -- blame it on the wine I had last night :bottle: . never messed with prepaid credit cards.