I wrote a five part post series on maximum energy and whether you should buy it. It turns out the answer is very complex and depends a lot on how you play.
I personally don't buy up my maximum energy tank, but based on my own numbers I think that is a somewhat irrational decision at this point. If you'd like any advice on your own situation, please feel free to ask. Enjoy reading:
Maximum Energy Basics
Full Energy Refills
Energy and Mood, Apart at Last!
The Final Four
Should You Buy Energy?