Trader Joe's has three strengths-- first, they are good at figuring out where the American diet is going, and getting in early with reasonably priced versions when the grocery stores are still either not stocking those items or pricing them as specialty goods (so they were doing organic early on, I was finding things like quinoa and chia when those were still pretty unusual, TJ's was one of the first places I saw the coconut oils and flours that paleo uses, etc.). Second, they do a selection of good quality, well-priced staples/ingredients that are better than similarly priced basics at the grocery stores, or fill a niche (ie, the salted but not sugar PB mentioned multiple times above) that the grocery stores do not--they have a low fat feta that is much better than anything I've ever seen elsewhere, for instance, and in the fall and winter they do several coffees ground with spices that I love and cannot get elsewhere). Third, they do prepared foods and snacks that are different from what you can buy elsewhere, have mostly natural ingredients and are fairly reasonably priced. While eating those on an everyday basis wouldn't be all that Mustachian (or healthy), many home-cooking Mustachians like to keep a few convenience foods or special snacks or condiments around to stave off the temptation to eat out. The combination is genius--I don't think I'd go there to buy from only one of those categories, but there's enough I get from all three that a trip is worth it every couple of months.
They used to change out products more often so it was tempting to go in there just to see what they have, especially when the new flyer comes out. Nowadays, they seem to stick with a more standard selection and do different seasonal variatons--so right now they are doing mango versions of their cookies, cereal, granola bars, frozen foods, etc. In October, there will be pumpkin cereal, granola bars, cookies, etc etc.
Other things--so-so. Their produce is usually pretty meh and goes bad fast, some of their branded products aren't that tasty, etc.