My 22 year old sports car recommends premium and that's what I put in it. It gets driven 2000 miles a year though (some years less), so its not a big deal. My other cars get regular.
2.99 for regular, 3.49 for premium here in Jamison, PA today. At .50 more a gallon, unless my math is bad, I'd have to get 4+ MPG better to make premium worth it. I've never seen any where close to that kind of gain, but I'll admit to not running controlled tests.