For years I've belonged to Costco. I don't buy as much there any more - not because of savings - but because you must buy in bulk, spending more at the time than if you bought a smaller quantity. I vaguely remember college finance courses and the time value of money. In other words, there is a cost to your money, real and opportunity. If you invest it, invest it early before you spend. But don't spend in advance. You could save the money and invest it and earn interest. In more practical terms, especially in this economy, you may not be able to spend more now even for savings - the money just ain't there.
Now, I wanted to see if I would actually save money even though I had to buy a membership for $50. First, I looked at their gas. It's cheaper by about $.05 - $.07 than the discount places I usually go (about $.10 - $.12 over the big name gas stations). In order to break even saving $.05 a gallon, I'd need to buy 1000 gallons. Since I normally buy about 18 gallons a time, that means I'd need to purchase gas 55 times to break even. In a year, I'd probably buy about 45-50 times. So that saves some but not the whole fee.
Lately, I've been taking over the counter medication. At Costco, they sell 3 weeks supply at the same price for 2 weeks at a leading chain. In that case, I'd break even over my membership after 12 weeks.
So between these two items alone, I feel that my membership is worth it.
Now, I haven't factored in the time value of spending the money up front, but the savings look to be sufficient to make up for it.