Plasma Pistol is my least favorite weapon in the Halo universe.
First of all, regular shots do nothing, making the primary fire completely useless. You never, ever see someone fire regular plasma pistol shots. Is it even possible to kill someone with it? Maybe if you've got 5 minutes and they're standing still. Why make it so useless?
Bungie listened to the complaints from Halo 2 and made some subtle adjustments to the PP that make it less annoying... most notably lowering the tracking and the ammo depletion while you're holding a charge. These are both good things. Still, this one shot takes away *all* your shields leaving you one headshot away from death. If your enemy is in tracking range, there's still nothing you can do. I haven't been killed by the PP/BR or PP/weapon X that much (especially compared to Halo 2!) but it still seems like a cheap weapon. And yeah, it ruins VIP. I mean, you can stick the VIP with a spike grenade, get a direct sniper headshot, or a direct laser hit and the guy will still be alive. ONE plasma pistol charged shot, though, is all it takes to bring him to no shields status?
The added function of stopping vehicles is cool but in practice not really a big deal because it doesn't stop the biggest threat of all -- the warthog turret. if you get close enough to PP, you're close enough to get mowed down in seconds.