Here's how we figure it -- the Sam's Club is on our regular path now (after 12 years of driving 13 miles one way to get there), so when we go, we can literally just run in to get two gallons of milk and not feel like we have to look around at everything.
Milk at Publix: $3.49 / gal.
Milk at CVS: $3.09 / gal. (which I just found out about)
Milk at Sam's: $2.89 / gal.
We go through 12 gallons of milk each month. Between the price at Publix and Sam's, that saves us $86.40 a year. Now that I know we can get it at CVS for less than Publix, our annual savings could be considered to be $28.80.
1/2 gal OJ at Publix: $2.50
1/2 gal OJ at Sam's: $2.24
We go through 8 1/2 gals of OJ a month. Savings: $24.96. So you can see, just between OJ and Milk, we have saved more than the price of the membership.
Our Sam's Club also has gasoline which is generally 3 - 5 cents per gallon less than the other gas stations between our house and Sam's. Sometimes it is even as much as 9 - 11 cents if we catch it at just the right time. But even at 3 cents per gallon less, figuring conservatively that we put in 12 gallons at a time twice a month times two cars times 12 months - that saves $17.xx a year.
The biggest thing about Sam's is knowing what things cost elsewhere and using your calculator to get your unit prices. And not buying stuff just because it's there.
Like someone else mentioned, get a day-pass and go see for yourself. Take a paper / pen / calculator and allow yourself plenty of time to go through the aisles to see what they offer and how much things cost for things you normally buy.