You end up buying things you don't need.
Yes, you might save money, but you need to have the room for buying in bulk. I honestly don't have room for 48 rolls of toilet paper unless I start putting it in different rooms...which is just silly...and my girlfriend and I will never go through a gallon of ketchup. I used to buy bagels, but every time I had one, I'd think "Hmm...even though these are Noah's bagels, this is just making me crave a real & fresh Noah's bagel..."
Yesh, they have electronics, movies and books...and sure their regular price is a bit cheaper than, say, Best Buy's regular price...but they don't usually compare to decent sale prices everywhere else, or even Newegg.
Yes, you can bring one guest any time you want. They don't "check" as long as one person is holding up a card when you walk in. They do frown upon multiple transactions. They've done it, but you can tell they get annoyed.
We were members for two years. I found that I bought WAY too much when I went there, and the prices started to creep up. For instance, Odwalla OJ used to be $4.49 a gallon. It's like $7-8 now. Once I realized the only things I bought there were Shampoo and conditioner, 3-packs of deoderant and 36 packs of Mt. Dew, and the occasional tray of wraps for a work thing... savings just weren't there to justify not only the membership fee, but the fact that I'd have to make a special trip just for those few items and deal with the parking and the idiots who shop there who act as though they've never pushed a cart before in their lives.
I mean, The bottle of shampoo I was buying turned out to be 4 cents/oz cheaper than at Target. Whoopie. I did the math once, and I figured I'd have to buy exactly what I was buying just to break even. Personally, I'd rather not go there, and pay the extra little bit and not have to make a special trip to Costco just for those things.
If you share your living space with 2 other people...will you buy toilet paper? Are you going to split the 48 rolls, 16-each, and write each other checks? Or will you each have your own 48-pack.
If it were a family of 3, and all the money was just coming from a joint account, or something, then it would probably be worth it.