UNC: Missing Marines

What he said. The first time you meet a Thresher Maw is a bit daunting. Just save before you attack and either drive around a lot or find a little rise you can hide behind and pop up to shoot.
And if you do leave it (I did first time) it's not the end of the world. Not a particularly important mission so don't sweat it.

Depending on what level you're at (or if you want to just leave the mission until a little later in the game rather than skipping entirely), you can also opt to take the thresher maw out on foot. In some respects, this is easier than doing it in the mako (depending on how easy you find the mako to control and fire at the same time). Why on earth do I say this? Because you can use powers as well as your weapons.

Depending on your class, take squadmates who use assault rifles and/or who have warp in their repertoire. Warp will make the maw more vulnerable to damage. Park the mako behind a knoll at the edge of the area and start to walk towards the vehicle at the center (on Edolus) or just to the center of the maw field.  As you near it, you will start to hear the ground rumble and the thresher maw will pop up out of the ground usually a little ways ahead of you.  I have never had one come up right under me while on foot.   Maintain that distance (don't get too close and don't back off) - this way the maw should stay above ground. IMMEDIATELY hit the maw with a warp and IMMEDIATELY start moving in a circle around the maw sideways (i.e. while still facing the maw). Continue to move while firing continuously on the maw. The maw will fire deadly acid at you, but if you just keep moving sideways around a circle in one direction, that acid will 9.9/10 miss you completely. You may lose a squad mate or two as they lag behind you. Revive them though if you can do so while still moving and if you need them to give you another warp. The concentrated fire is easier to keep up if you have the Spectre Gear (HMW weapons) since they don't overheat as readily (that's why I'm suggesting you do the mission later on in the game). Also, after you've levelled up and with HMW gear and weapons mods, you can actually do more damage per second than with the machine gun on the mako. Even on insanity, I am generally able to take down a maw before I've made one complete circle around it... and I don't bother even trying to use the mako anymore.

The mission is not mandatory, but it does unlock a couple of other side missions in different systems... and you get a TON of XP and credits for taking down the maws on foot.  Also, several of the side mission planets do have maws on them.  You don't have to kill them... there are, I believe, only two that are directly attached to side quests.  For the others, you can just drive quickly out of the area and forget them... but you do lose out on that XP and credits I mentioned.

I'd suggest doing the mission because it leads to some background info on an organization that becomes more important in ME2/3, plus the dialogue scenes with and about Kahoku help add a little depth to the Alliance.

As for the Thresher Maw - when in doubt, just keep driving around. It will spit acid at you, but I don't think it's that great at hitting moving targets. Blast it with your cannon, then if possible fire at it with your machine gun for minor damage while waiting for the cannon to cool down for another shot.

The quest isn't mandatory, but it leads to events which get mentioned by Miranda in ME2.

Yep, I get the same, with the possibility that the player will only encounter 6, 7 or 8 (depending on which, if any, morality quest(s) they unlock or do (because they can also turn down the paragon one); and/or of course, whether or not they land on the planets without the quest actually being unlocked).  Also,