The rocket launcher is a great piece of work, and bags of fun of course. Nice launching noise, and the rocket seems to fly absolutely straight, with perfect accuracy. Luckily you can carry a whole load of spare rockets too, so you can dish out plenty of hurt before you run dry.
The standard tip for using the rocket launcher against personnel is to aim for the ground just ahead of the enemy, rather than at their torso or head. You could easily miss a body shot if they're moving, whereas the blast from a ground shot should still do plenty of damage even if it's not as close as you'd like. That said, if you're sure the target isn't going to move, or is going to keep headed straight for you, a shot direct to the body scores pretty highly on the satisfaction scale!
Distant moving targets
It's tricky to hit a moving target a long way off of course, but it's also a nice challenge to pick the right moment to send in your shot after watching the enemy behaviour for a while (this is 'patterning' for the rocket launcher, a practice more usually associated with the sniper rifle). Thrust-firing will send the rocket in faster, which should help.
Naughty aiming trick
If you've got a sniper rifle, you can aim with that, then switch to the rocket launcher so it'll be aimed where the sniper rifle was, for spot-on accuracy. Tempting though it may be, I prefer not to use this trick in serious play as it doesn't really map to reality. It's sort of a cheat that only works because of a programming quirk - a glitch you might say. If Bungie wanted to make things more realistic, your aiming spot wouldn't be exactly 'remembered' in this way when switching weapons; it would be randomized a bit. However, when I'm just having fun, I don't mind using this trick to aid some bit of mischief or other (usually involving something nasty happening to unsuspecting enemies way down below a high ledge I'm perched on in level 5).
If you love the rocket launcher you should get into the spiral path megabattle; it's a rocketer's paradise. I often use thrust-firing in that scenario when trying to take out enemies heading towards or down the spiral path, when I'm down below. It reduces the rocket flight time, making judgement of where to aim slightly easier as you don't have to aim quite so far ahead of the enemy. The twin bridges megabattle is another great playground for the rocket launcher.