I spent part of last Thursday at Costco in Schaumburg, Illinois. (My wife and daughter spent THE DAY at Ikea and I didn't want to endure all of that).
At any rate, I went to the Costco and asked if I could look around without being a member. The gal at the counter was very polite and gave me a membership form. She explained that I could look around, but if I wanted to purchase anything I would need to become a member. Cost of the yearly membership was $50. After spending a couple of hours looking carefully at many items, I decided it wasn't in my best interest to spend the $50 membership fee. Perhaps part of that thought process was the long distance for us to drive to the nearest store (150 miles).
Like others have posted, the quantities are generally very big and don't fit my particular needs. I thought the prices were competetive on many food items, but nothing that would save large amounts of money for me.
On the bigger ticket items, I didn't see any savings at all. One item in particular was a 30gb iPod, which retails for $300, but can be purchased lots of places for $289. Costco had it for $289.
I have friends in the Seattle area that love Costco and claim they save hundreds of dollars each year. However, they have a large travel home and spend 1 month in Baja Mexico each fall and another month in Galveston, Texas each spring. They load up with food from Costco and keep it in the freezer and fridge in their motor home.
So I guess my observation is that Costco can be a real savings if your life style fits what they have to offer. Our life style doesn't seem to save a great deal, but they were certainly polite and nice to me when I was there last week.