Rune Master-

The main point of this perk is to make Rune’s incredibly more usable.  Most of the time under normal conditions you’re stuck either sneaking around, or running like a frightened deer and hoping your enemies stumble into the rune you left behind either as they patrol or when they chase you.  While that -is- how traps work, it’s not the best way to function in a game like Skyrim, so if you really want to use your Runes more, this perk is fantastic.  If, on the other hand, you don’t bother with the things in the first place and they don’t suit your style much anyway, just skip it.  It’s not required for anything interesting, and it only affects the Rune spells- a small portion of the power available to a Destruction Mage.

Intense Flames-

This is possibly the most aggravating perk available in the tree.  Conceptually, it’s hilarious and useful- set someone on fire enough and watch them run around like a screaming chicken-torch until their health burns away and they keel over.  Effectively, it’s only like that about half the time.  The rest of the time, it’s aggravating- fleeing enemies have a tendency to leap off cliffs or into areas that are difficult to enter or leave, making collecting your loot from their charred remnants vastly more difficult.  That said, it can save your butt in a narrow fight, though not as well as the other two equivalent perks.  Take at your own risk.

Deep Freeze-

A vastly superior option to Intense Flames, Deep Freeze makes ending battles slightly more tactical- a paralyzed enemy won’t stay that way for too long before they thaw, so you do have to get back to them, but while they’re stuck you can turn your attention to something more dangerous, or even take the chance to patch yourself up a little before something else runs in to chew on your leg.  While it’s just not as fun as watching someone run around shrieking ‘OHMYGODIMONFIRE’, not needing to chase your loot down a cliff or back down the hallway you just came here from is considerably less stressful to begin with.  Also, a frozen enemy means a much safer end to a narrow fight and keeps you from having to worry about a lucky critical hit or sudden stumble.

Disintegrate-

Possibly the best of the three elemental low-health-target perks, Disintegrate pretty much instantly ends a fight when your target gets close to dying, completing the job absolutely free.  The slightly lower effect of lightning spells on non-caster targets is pretty thoroughly offset by the usefulness of not having to fight the last eighth of every single battle ever.  Turning things to ash can get a touch annoying when you have trouble finding the ashpile you haven’t looted yet, but that’s a tiny drawback for a perk that’s so useful and so visually impressive.