If it's a century, I don't take any solid food - just 2 sachets of Perpetuum that I mix with water on the road, and 2 gels (Gu or whatever) for the last hour.

For a longer ride, such as a 200km (130 miles), I'll take a few oatmeal bars and 2 more Perpetuems, and make sure I finish them all.

For still longer rides, I pick food up along the way. I don't want to lug all that extra weight around (why burn calories to carry calories? ) and I don't like being encumbered with the bulk of solid foods.