I like the ones with added caffeine (e.g., Strawberry Vanilla Power Gel). The carbs in gels get into your bloodstream really quick, and the caffeine provides a nice psychological boost.
When I'm riding in the big mountains, I always have a gel shot just before the start of each major climb. It may just be mental, but I find it helps.
They're also useful if you screw up and get behind on fuel, and can make the difference between getting home OK and a torture session with the evil Dr. Bonk.