Trader Joe's has managed to maintain low prices by carrying it's own brands and labels (including Trader Jose's (Mexican food), Trader Ming's (Chinese food), Baker Josef's (bagels), Trader Giotto's (Italian food), Trader Joe-San (Japanese food), Arabian Joe's (Middle Eastern food), Pilgrim Joe's (Seafood), JosephBrau (Beer), Trader Johann's (Lip Balm), Trader Jacques' (imported French soaps), Joe's Diner (certain frozen entrees), Joe's Kids (Children's food), and Trader Darwin's (Vitamins)), keeping it's stores smaller and keeping the set-up as minimalist as possible. This approach has been so effective that they manage to beat Safeway by a wide margin on prices for many common items.
Spreadsheet Comparing Prices on Products
Here are some images from the spreadsheet, but it is hard to read so I defer to the link above.
For my most generic needs or certain brands, like paper towels or Jiff/Skippy peanut butter, and sometimes meat, I'll go to Safeway. Mike was right when he said that "[Safeway] is a grocery store," in a way that Trader Joe's will never be. But if I want to assuage my ethical and sustainable pinings through buying organic and free trade and environmentally friendly without having to pay through the nose, or if I really want frozen mango chunks, I go to Trader Joe's
Trader Joe's - Foggy Bottom (4.5 Stars, 116 Reviews)
Safeway - Georgetown (4 Stars, 6 Reviews)
Note: Keep in mind that these stores happen to fit my personal model of shopping. These prices and products (which are just a sampling) have been compiled due to a manic need to maintain a reasonable budget for groceries, otherwise I'd be fat and broke. Though it should be noted that the prices at either store are not astronomically high by any means.
If you are serious about budgeting, then save your receipts and compare prices and keep a spreadsheet/pricebook/etc. But for the most part, a regular grocery store, a la Safeway, can, for the most part, do you no wrong.