We signed up for the first time a few weeks back. The reason was because the tires we desired to buy (nice michelins) were cheap enough that the amount we saved compared to tire rack or any local place already paid for the memebrship.
If you go shopping there blindly, you may not save as much money. Their stuff seems to be nice and good quality, but some stuff just isnt cheaper. Case-in-point, we were having a party and wanted to make BBQ pork. Wife bought a bunch at costco. Turs out it was cheaper at the local supermarket.
However, other stuff can be a steal. We were able to buy 32oz containers of raspberries for less than the tiny containers cost at the supermarket.
Some stuff is hit or miss. They dont carry all grades of M1, and the cost without coupon is higher than WM or a deal at the parts store. Their batteries are a good price and highly rated, but they dont offer every size, in fact they dont offer any sizes relevant to me.
When you go, you tend to pay more then you would at a supermarket, but what you buy is usually greater in quantity, which explains it. The per unit price may well be much cheaper (but as mentioned, not always).
We have been happy with what we have bought, but time will tell if year over year the membership is worth it.
If buying on a CC is necessary because you cant afford what youre buying, then you should rethink your budget and what youre buying. Rewards cards are nice, but nobody has ever gotten rich from spending their way to freebies on a credit card. If you really get into costco, the AMEX or one of the cash back debit cards may be smart. If the local costco sells gas, buying an executive membership gets you 2% back, and then the amex gets you another 3% back on gas, which isnt bad...