Well, all the energy-based champions have 200 energy and 50 energy regeneration per 5 (or 80 energy regeneration per 5 with blue buff).
You can get up to 42 extra energy if you get energy per level and flat energy, or 36 energy with just flat. With energy regeneration per 5 you can get another 10, or 15 if you invest in the per levels. So, you can increase your energy pool by 18%-21%, and/or increase your regeneration by 20%-30%, which seems somewhat substantial.
Only problem the runes run into is weather or not you actually run out of energy often enough for them to make a difference. With Akali, that's generally not going to happen. I've heard with Shen and Lee Sin it can be problematic though, so with those champion you may want to play around with the energy a bit, if you have the IP to afford them. For a rather niche rune that only effects 4(?) champions, they are obscenely expensive though.