I would agree that in pvp it is easy to run out of mana. I find myself spamming chain lightning every time I have the chance. This will eat away at your mana pool pretty quickly.

Only times I ran into mana problems in PVE was when I first dinged 70 and did kara. Since then I haven't had a problem. I was messing around with the training dummies last night too see how much dps adding a mage would bring. I was timing how long it took for me to go from full mana to oom just casting lightning bolts. It took close to 2 mins to empty my mana pool. Heroic boss fights were usually 15-30 seconds for me.

Unrelenting storm isn't a bad talent. I just wouldn't take this over a talent that will increase your overall dps.