Except for produce, I've never had any quality issues. In most cases, I've found the Aldi brands to be equal or superior to the top-selling brands.
Aldi also has a double money-back guarantee, so iff you're disappointed in a product, they will give you back twice what you paid. We returned a bottle because the seal was missing. They offered us the choice of double cash refund, two replacement bottles, or cash and replacement. We took two mustard.
They're sweet potato chips are awesome. So is their fish and shrimp.
For the most part, I only like mild fish, and what we've bought from Aldi has been every bit as good or better than the same type of fish bought anywhere else.
Our local store has dramatically improved the produce quality, selection, and price. We don't buy much produce there because (in the summer at least) the farmers market is cheaper and better, but there are exceptions. We bought some astonishingly high quality asparagus last trip for about 1/3 the price of the grocery store nearest our home (and their prices are lower than most).