'Can I REALLY skip the priming?'. . . that's a question a lot of homeowners have about so-called 'paint + primer' products. There are several interior and exterior waterborne paints that have self priming properties on the market--including Benjamin Moore's low-VOC Aura and Regal Select paint--promising one to two coat coverage, over most surfaces when used without a primer. Using products like these, you can expect to cut painting time in half on some projects. As a general rule, these paint + primer products are a good choice when you're repainting work in good condition.
Paint and primer play very different roles, of course: resins in primers are designed to adhere to the substrate and seal it; paints have more solids and go on thicker creating a protective 'skin.' These roles can be combined over some substrates, but others need a separate primer, so the substrate and current conditions should drive your decision on whether a primer is needed or not.
Bleeding woods like redwood or cedar, for example, have tannins that will react with water-based paints, so they should always be prepped with a primer designed specifically for tannin-blocking. Water-stained surfaces are also tricky, and need sealing with an appropriate stain-blocking primer. Glue residue from wall-coverings can interfere with adhesion, so walls that have had wallpaper removed from them should always be primed. If you're painting over bare ferrous metal, moisture in the paint can react to iron in the metal and cause flash rusting, so these surfaces should always be coated with an oil-based primer designed for metal. Even galvanized metals perform best when protected with a corrosion-resistant primer such as Benjamin Moore's P04 Acrylic Metal Primer.
As you can see, there are plenty of exceptions to the 'no-primer' promise, so check labels and technical data sheets (usually available on the manufacturer’s website) carefully to find out the limitations of the product that you're using. Self-priming paints can be a real time saver when re-painting a room a new color, painting wood trim (non bleeding woods), and even on new sheetrock or repair work. Every project has its own recommendation, so before deciding to prime or not, read the label instructions to assess if your job fits into a self-priming category. If in doubt, contact the manufacturer for their professional recommendation.