How do I use the grenade launcher effectively

I've earned the grenade launcher in Alien Swarm, but I really can't find a way to use it effectively.

It seems to be very useful for thinning the hoards in Onslaught but there always seem to be some stragglers. Because of this I can't really see it as useful to those on point (nor rearguard) as the autogun (or minigun) have a considerably more thorough covering arc.

Has anyone used this weapon successfully?

The grenade launcher is one of my favorite weapons. It's wonderfully effective against most enemies and shoots incredibly quickly and precisely. Note that grenades hit where your targeting reticle is, so you can adjust the distance you shoot that way.

It's particularly useful against:

• Large groups of drones, parasites, or those harvester spawn things, since it takes out all of them in a single hit.
• Rangers, because you can easily reach them, even on ledges
• Harvesters, because spamming grenades will easily kill the spawn it releases on death.
• Mortarbugs, because the grenade launcher deals massive amounts of damage in a very short time
• Eggs, because you can quickly saturate a whole room with grenades.
• Doors, like BlueRaja mentions. The chainsaw is a lot more efficient though.

It does have a few disadvantages, but none crippling:

• It doesn't hold a lot of ammo. Just take an ammo kit as your secondary, and you'll be fine. Ammo kits replenish a very large amount of grenades.
• It does a lot of friendly fire damage. Don't fire near teammates. Staying in front helps.
• It does a lot of self damage if you shoot too close. Run behind your teammates if enemies get too close. Taking a strong melee character like Vegas can help tackle small numbers of drones.

It's an awesome weapon.

My friends and I used grenade launchers together for blowing down doors without having to hack them, along with the flamethrower (which is incredibly overpowered if you burst-fire it) and adrenaline, to get all the speedrun achievements. We used this setup on most, but not all, of the levels.

The chainsaw is also good for this purpose.