I've never tried energy tablets, but did take energy gels on my long runs when training for the marathon last year. I timed taking the energy gels with every other water station on the course, so took one every 10km.
I don't bother with gels/tablets the rest of the time, just water on the longer runs of 8-10miles or more.
I don't really like energy tablets for running, not sure why just seemed to make me cough rather. But I have used them while hiking and found they did make a difference, but the effect didn't seem to last long at all.
it's basically just simple sugars that'll give you a short burst of energy when energy levels are low on longer duration runs etc... you'd never notice a difference in short distances but in marathons etc it would probably mean the difference between hitting 'the wall' and not... which would have a massive effect on your finish time.
In reference to the last comment, I should add that I don't mean energy tablets must be eaten during a marathon, you could eat a whole number of substitutes (gels, jelly beans etc), but some form or nutrition/energy source in general would be desireable to avoid hitting the wall.