One of "those". Like the MP-3 of Rainbox Six, the home-run bat of Super Smash Brothers, or the Redeemer of Unreal Tournament, the pistol is one weapon you'll be hunting dark alleys and rotten corpses to find (well, you start many levels equipped with one, so not literally... but you get the idea). Although it's not the most powerful weapon in the game, it's close, with - for those who care - an impressive .50 caliber Covenant- and Spartan-stomping round that'll clear out your oppressors in no time. It'll fire near as fast as your finger will pull, but it's considerably more accurate if you resist the temptation and give considered single shots. An excellent device for clearing out unshielded elites, and pretty much anyone else, too - Grunts are surprisingly vulnerable to it, it's a good choice for zapping through shield firing-ports to upset Jackals, and it's one of the best Hunter-fighters in the game. Also features a handy 2x zoom that's just as useful for half-assed (but effective) sniping as it is for getting a binocular-esque take on a situation.
You shouldn't have much trouble finding ammo as long as you're frugal with your fire, and even those with profligate trigger-fingers aren't likely to have difficulty unless you're in a very in-Human area (although it should be noted that dead Marines can crop up in the strangest places.)
The melee attack isn't the worst in the game, but it's not something you'd want to write home about.
A cyborg's best friend. Just don't tell Cortana.
Pros: Fast rate of fire, good accuracy, powerful cannonballer shells, 2x zoom, great compliment to many other weapons
Cons: Fairly slow melee, requires some small amount of accuracy, looks sissy in front of your friends in Multiplayer, difficult to find "Cons" for in Halo Weapons summaries.
Usage: A wondrously good back-up. Drop an enemy's shields with a volley from an assault rifle, shotgun, plasma grenade, etc., then switch to the pistol and level him. Useful in nearly any situation, so don't get picky.
For: Anybody with several working brain cells. Has a shallow but protracted learning curve; beginners can have a ball with it, but in the hands of a master, the pistol can sing, dance, and tell campfire stories. Beware the shadow that holds a pistol.