The rest of the Armor Abilities are really straightforward, but the Thruster Pack just appears to sputter about for a few seconds when I activate it (instead of being "a powerful burst of movement offering the ability to evade or quickly close the gap"). Am I doing something wrong?
The Thruster Pack, when activated, makes you move quickly in the direction you are holding for a second or so. It can, as described, be used to quickly get close to an opponent or to dodge attacks.
Your problem may have been that you were not holding any direction when you pressed the button to activate it. Since it wouldn't have had a direction to move you in, you wouldn't have gone anywhere.
In some gametypes, the Thruster Pack has a dramatically reduced boost. You're not doing anything wrong. EBongo brought up the flood gametype; in that the Flood team's boosters are much more powerful. There is a silver lining to using the Thruster pack in normal games though; If you activate it as an enemy is assassinating you, you break free of the assassination!
I was also fairly dissapointed when I used the thruster pack in War Games and Campaign and other armor abilites are more likely to take it's place. I do have one loadout with it, a plasma pistol, and plasma grenades as an equivalent to Halo: Reach's Armor Lock - if an enemy ghost or vehicle is lining up to ram you it's possible to use the thruster pack to evade them. It requires a bit of timing i.e. at the very last second, or it would be possible for the enemy to change their course and splatter you. After it goes past I used a charged plasma pistol to disable it, and stick grenades to it before it re-activates. If you have a sticky detenator it's even more satisfying to stick the vehicle and detonate it a couple of seconds later.