If you are looking for a big purchase (TV, furniture, etc), then it all comes down to whether the $50 +$Purchase Price is a good deal. If it is, go do it (that's how I got it, they had a good deal on a 42" LCD TV back in 2008).
If it will mostly be grocery purchases its a little harder to justify for one person.
Their milk is consisantly $1 less per gallon than most stores. The exception (around here anyway) being Aldi, however I'd trust Costco milk (from non-hormone injected cows too) a million times more than Aldi milk.
Don't get me wrong, I love Aldi, I just wouldn't trust them with things that can go really bad easily. I use them mostly for fresh fruit and veges where I can easily see what quality they are.
Anyway, a gallon of milk a week already pays for the membership.
Prices for other items vary. If you wait for coupons you can usually get cereal for really cheap, and that lasts forever so you can buy in bulk. If you bake your own bread (which if you are looking to save money and have great tasting bread, you should) you can buy 2 pounds of yeast for like $4 (which is a TON cheaper than anywhere else). Similar deal with flour and olive oil.
Things like soda really aren't a good deal. For cans it comes out to 23 cents a can or so is their regular price. Around here sales for 12 packs at regular stores (Target, Jewel, whatever) go for $2.50/12, or about 21 cents a can.
Their meat selection is great. You can get prime grade top sirloin for $6.99/lb (used to be $5.99/lb a few months ago

). Its a very tasty/tender steak for dirt cheap.
Some things are always cheaper at Costco, somethings are cheaper than non-sale grocery store prices, some things are more expensive.
Anyway, in short, it may or may not be worth it depending on your needs. Why not take a walk around the store to check it out? Also, you can return your membership and get a full refund at any point. So you could even try it out for a few months to see if it actually saves you any money.