Majestic and gorgeous, for me these things are quite a highlight of the game. I love seeing them cruising around, and the deep humming sound effect adds a sort of ominous atmosphere. That said, it's a bit comical the way the occupants are squashed into their compartments. It doesn't look terribly comfortable, and I can't help thinking it must be a bit drafty inside too as there's actually a gap where the door closes, which you can potentially sneak a sniper bullet through, causing an enemy to die and impossibly fall through the floor.
The dropship also has something of a defensive flaw. There's often little need to hide from its gun as it seems so poor at spotting you, even if you're standing out in the open right below. However, if you open fire you can expect a faceful of pinkness in pretty short order. Unfortunately there's no actual point in firing as the ship doesn't take damage (don't you wish it did!), but sometimes I'll fire anyway, just for the hell of it. Hitting a moving ship with a rocket is satisfying, even if it actually does nothing.
Blitz those Elites
The main danger from a dropship is provided by the Elites inside, if any. In practice you tend to know which side they're going to be on (unrealistic of course, and I do wish things were less predictable), making it fairly easy to blitz them as soon as the door lowers, before they've even hit the ground. Two options there would be to tag them with plasma grenades or blast them with plasma weapons. Killing a pair of such Elites is trickier, but you should still manage it if proficient with your weaponry. Once the Elites are dead, the rest of the enemy will head for the hills, and it's open season.
If you want to play more realistically and not rely on impossible advance knowledge of the enemy's configuration, you should oblige yourself to keep a suitably cautious distance from an arriving dropship, until the troops are out. It's surely what you'd do in real life, since for all you know the ship could be jam-packed with Elites. That'll probably make life a bit tougher of course.
Amusingly, Grunts occasionally somersault to the ground, which is fun to watch out for if you're a fan of the little fellas. See Somersaulting Grunt in the fun section for a bit more about this.