Back when the new edition hit the gaming clubs, the Giant looked like everyone's favorite whipping-toy. As enemies got to strike back at all times, suddenly that no-save toughness 5 wasn't feeling very stable, and an almost unfathomable (I'm sure) amount of Giants were slain and subsequently dropped from just about everyone's army lists during the first couple of months of the new game. Now as things progress, more players are figuring out how to make good use of Monsters, even the big beasties who won't even regenerate their own wounds.
If used correctly, Giants provide something very unique to our army and can prove to be one of the most devastating forces on a battlefield. The key to that, more so than most other units in our arsenal, is to make sure the Giant is fighting what he should be fighting and staying away from things that he shouldn't be fighting, at least not on its own. By himself, the Giant is amazing against tough yet slow 'big things' that neither Orcs nor Goblins can do much against. The Steamtank fits that bill. With its toughness of 10, 10 wounds and 1+ armor save, even the can-opening Black Orcs will take a long while to get through it, and the Trolls have to vomit for several rounds before the Steamtank will begin to really feel it. The Giant, on the other hand, has a 50% chance to automatically inflict 2D6 wounds without armor saves allowed, every round of combat (you want him to thump things with his club) which should make short work of the Tank. The same can be said about the Skaven contraptions (Screaming Bell, Doomwheels and Plague Furnaces) which will all fail their dodge tests if the Giant thumps them, where the big guy can be a strong key to defeating the Skaven. If you're feeling brave or desperate (or both) you could do well in sending the Giant up against a Hellcannon or even a Hydra, but you should expect to take a few wounds in return and you have to get the right attacks from the big guy. Stay away from the Abomination though, as that thing will chew right through you.
However, the Giant will cause the very most destruction when supporting your infantry units against other infantry units, even those with a toughness of 4. This can either by flanking the enemy, which is fine but can be risky, or more commonly by ranking up corner-to-corner at the front with an enemy unit already engaged with your own infantry unit. Since Giants don't need (or even want) to maximize their own facing against an enemy in order to dish out the damage, they are quite happy in sweeping their club across the enemy ranks while limiting the amount of enemies that get to strike back at them. You'd much rather see the enemy direct their attacks against your expendable foot sloggers than hurting the Giant, since as long as your infantry provide the needed ranks your Giant should inflict enough hurt to win you the combat.
Preferably, you'll want your Giant to support charge next to Night Goblins or Squig Herds, for the simple reason of initiative. Although Black Orcs or Savage Orcs, when paired with a Giant, dish out so much pain even Gork would be impressed, the Giant will strike ahead of them in combat. Therefore, you risk the chance of the Giant chosing to Yell and Brawl, immediately ending the combat and wasting your choppa bonus, not doing close to the damage you should have been doing. On the other hand, since Night Goblins and Squigs strike at the same time as the Giant will, it's a good idea to roll all of their attacks before checking what the Giant will do. This will maximize the pain you're dishing out, as although gobbos might be more useful for their ranks than their combat proweness, even the most novice general will realize you'll kill more enemies if you strike at them than if you don't. A Night Goblin mob with Nets effectively makes your Giant have a toughness of 6 so it's safe to say they make for a good army synergy on the battlefield, with both sides aiding one another. Although the gobbos will lose more of their own than they will ever kill of the enemy, when your Giant is adding an additional 3D6 S6 hits for your side (by Jumping Up and Down + Thunderstomp) even Chaos Warriors will begin to worry.
As long as you realize where your Giant will do the most good for your side, he can be a very devastating and valuable asset to an Orcs and Goblin army, and easily makes up for his point cost. However, he will go down to cannons and bolt throwers without much trouble, and will hesitate to go up on his own against infantry units with better Weapon Skill and a Strength sufficiently high to wound him on a consistant basis. That being said, he is definitely a lot of fun to field as you never know for sure what he'll be doing each combat turn, which alone might be reason enough to bring a Giant to the battlefield.
Unit upgrades: If your army includes a Savage Orc Shaman, it could be a good idea to give your Giant some Warpaint to keep him safe, although it is a bit on the pricey side. You know how annoying it is when those pesky Skavenslaves, or Night Goblins, keep making their parry saves even though you're hitting them with a strength high enough to wound a Steamtank? The same can be true for a tatooed Giant. Bear in mind that it's just as big of a chance for that single poisoned shot from a Skink javelin to wound you as it is for your Giant to wardsave it. Just make sure that you do have those points to spare, as it does mean almost one less fanatic in your army for your Giant to have a 1/6 chance of warding off an enemy wound.