As has been previously noted by CHOW, "natural" peanut butter (just peanuts and maybe some salt, no other additives) is just filled with flaws. It's gritty. It's hard to spread. It separates into dry peanut mash topped with a slick of oil that's messy and a pain to stir in.
Cook's Illustrated has taken up the matter of separating spreads in a story in their March/April 2011 issue, "The Reinvention of Peanut Butter" (only part of the article is reprinted online).
Turns out that the problem is an old one: "The earliest mass-produced peanut butters didn't contain any added fat," writes Diana Burrell. "But just like the single-ingredient peanut butters manufactured today (and the ones you can grind yourself at the supermarket), their texture was so stiff from the inevitable separation of oil from the ground-up nuts that grocers received the spread in tubs with instructions to stir frequently with a wooden paddle."
A chemist named Joseph L. Rosefield cracked the separation problem in 1922 by replacing some of the naturally occurring peanut oil, which is liquid at room temperature, with hydrogenated fat, which is solid at room temp. Thus a more stable spread, which unfortunately has the side effect of not being nearly as healthy as just peanuts. Indeed, as Cook's Illustrated points out, many of the spreads now labeled "natural" on grocery shelves contain additives like molasses (not a big deal), sweeteners (not so great), and palm oil (loaded with saturated fat, not popular with environmentalists).
Putting aside matters of health, Cook's Illustrated eventually rated Skippy Peanut Butter as the top of the heap for creamy varieties. It contains hydrogenated vegetable oil, which is not the dietary demon known as partially hydrogenated oil (trans fats!), but it's still not great. The best crunchy butter was JIF Natural Crunchy Peanut Butter Spread. Notice the "spread." That means it's more than 55 percent fat, and less than 90 percent peanuts. And it has palm oil.
With all that in mind, my household is going to stick to the peanuts-and-salt varieties, stirring be damned. By a weird coincidence, I happen to have three bare-bones varieties of peanut butter in the house right now: Whole Foods' 365 Organic Everyday Value, Adams (made by Smucker's), and Trader Joe's Crunchy. A taste test by our family of three found us in agreement: 365 is gritty and stiff; Adams is runny; Trader Joe's is decently spreadable, tastes the most like fresh peanuts, and was our favorite.