It's the only console game I've ever owned that can reliably put me into a matchmaking game on any mode of my choosing in under a minute. You may already know this, but Halo 3 still gets a buttload of online play, so of course the newest incarnation is going to be perpetually populated. Factor in the constant support (after a year of solid play, I'm still getting into matches with rule sets and levels I've never done before), and you've got my reasoning for why I personally consider Reach's multiplayer to be on par with TF2's.
EDIT: Oh, and tips for getting good? Here's one:
Be an equal-opportunity user of all weapons. If you are bad with a certain weapon, force yourself to use it until you're better with it. Every weapon in the game has its strengths, weaknesses, and intended role; when used with a deep understanding of its mechanics, any weapon can be used to dominate. Think the plasma pistol is garbage? Actually, that little sucker will shred through shields even when firing normally, can EMP vehicles into locking up, and reliably deals damage that nearly puts it on par with the Magnum (except it doesn't lose accuracy).
Also, understand that switching weapons is huge. You aren't allowed to carry two weapons just so you can switch to the other one when the first one is empty. There are weapons that compliment each other enormously. The plasma rifle eats shields like a fat kid goes through a bag of cheetos, and can even take down the shields of multiple enemies in one well-controlled burst, but deals crap damage to health; on the flipside, the DMR is a weapon that takes numerous increasingly-inaccurate shots to down a shield, but as I'm sure you've noticed with the clusters of unfortunate grunts that think numbers will protect them, it can put multiple bullets in multiple heads before the reticule bloom even starts to even matter. Do the math.
