Adam - GA has a point about the concentration of carbs in gels*, which does make them handy. And there is definitely the psychological element - if you've got a gel / jelly baby to look forward to in x number of miles, it helps to break the race up into chunks. The only practical problem I had with jelly babies, which is why I've never used them again, is that they got stuck in the throat somewhat; this was London 2007, a very warm year. SIS Go gels are more palatable. But by all means try them. Just make sure whatever you use on the day is tried and tested, from every practical point of view, i.e. can you carry the required amount?, get to them easily whilst running race pace?, etc. (I had a minor panic on the start line of the Paris marathon when I realised how bouncy my running belt was with a gel stuffed in the pouch!)
*I like to think I can get away with drinking just liquids even if this isn't providing a huge number of calories, cos I'm pretty sure I'm a better fat burner than I used to be. (No energy issues after a 24 mile run on no breakfast, including 7 miles @ MP would suggest I'm fairly efficient.) That's another thing - whilst various people have strict rules about so many calories required per mile, or bodyweight or whatever, this can vary greatly between one person and another. And I sweat a lot, so liquid replacement with some electrolytes is a good thing!
Edited: 14/03/2013 at 16:17