I pretty consistently choke down one clif bar a day on a trip. For a longer trip and consistent high-mileage days (e.g. 20+ mpd), maybe two a day and a gel or two.
Probably the biggest thing I look for in a "food bar" is that it should not break teeth at low temperatures and not require a spoon to eat on a warm day. Clif bars do that, and come in enough flavors to ensure variety and I generally am happy with the flavors. Actually, the peanut butter flavor reminds me of homemade granola bars.
The caffeinated bars are great when you roll out of bed early and skip brewing up, and also on a midafternoon when you still have nine miles to go to make camp, or at the start of a 3000' climb.
I'm neither proud nor consistent, though, and have consumed tiger's milk bars, odwalla bars, snicker's bars, various granola bars, bear valley "pemmican" bars (although they have this aftertaste that makes me think of paint thinner).
A local bakery here makes "date bars" which are outrageously dense, and probably have about fifteen hundred calories. They also sometimes sell "granola bars" which are also quite tasty. These don't keep as well as the commercially packaged bars, but they are so much more tasty that it is often worth the trouble.
I've experimented with the various "gels" over the last few years, and found they are great for quicker energy (the Clif Bar buzz seems to take about twenty minutes to kick in, and lasts 2-3 hours) and for making you thirsty enough to drink more water. Supposedly they are good electrolyte replacers as well, and that can be important if you are drinking four or five liters of water a day. My big gripe about the gel products is the packaging is awkward. I'd rather see the stuff in a tube with a cap (like toothpaste, pesto mix, or wasabi) and I could just take a hit off the tube when the urge strikes. A tube that held the equivalent of 3-4 gel packages would seem to be about right. My guess is that it would also be cheaper to package them this way, and since the gels are so outrageously expensive that is no small thing.