have found many gels and bars go through my system like a ball of glue, not saying they're no good, love using them, however they must be used with adequate hydration.
My favorite are Torq, nice taste and not too over-powering.
I use gels etc, as a bit of a last resort treat usually at Mordi (going in anticlockwise direction). I reckon you want that boost they give for last section of ride.
If you are not using gels etc in training don't try new ones on the day, otherwise it's a burning gut for remainder of ride.