Orc Boar Boyz, as opposed to their more slightly more expensive savage brethren, are quite tough and heavy armored at 3+ save. They won't hit as hard with their single S5 attack, but neither will they be lured away by pesky diverters or overrun into unfavourable positions. Since they won't be frenzied, with all the benefits this means, they'll be hard-pressed to take on monsters such as the Hydra without the backing of a Boss of some kind. They should, however, still make a dent in the flank of an enemy unit and be able to chase them down if the break, thanks to their swift striding ability.
Small units of 5-6 can make for a decent unit capable of chasing down smaller missile units, war machines and skirmishers, that your blocks don't want to spend time dealing with, or even threaten enemy chariots thanks to their good speed. With their decent armor and toughness, they won't fear the stand and shoot reaction from most of the above and should make short work of the enemy on the charge.
However, contrary to the Savages, it can actually work out well to bring a larger unit of Boar Boyz to the field. If you're a fan of cavalry orcs, and there certainly are those out there that are, a unit of 10 Boar Boyz in two ranks, joined by a Big Boss or even the Warboss with a spear can make for a 'ard-hitting hammer unit that won't cost you very much at all. Where the Savage Orc Boar Boyz will lose out on 2/3 of their attacks when placed in the second rank, the Boar Boyz will still get their single attack (although the boars won't fight in the second rank) and have a good enough armor save to suffer through a lot of shooting. Once your infantry orcs have pinned the enemy in pace, your orc Hammer should hit their flanks and turn the battle in your favour.
Unit size: 5-6 Boar Boyz make for a very cheap and surprisingly resilient unit to hunt down smaller enemy units on their own. Units of 10-12 are still quite cheap and may function as a good, fast bunker unit to get your 'ard-hitting Big Boss or Warboss into combat wherever they choose.
Equipment: The additional strength bonus of the spear makes it a no-brainer choice for the Boar Boyz, and as they're supposed to be quite resilient, the shield upgrade as well is a must.
Command options: As they're fast striding cavalry, the Musician will help out a lot with swift reforms. The Standard Bearer will give you an additional vitality point for some missions and also increases the chance of you breaking the enemy on the charge. The Boss upgrade is only a good idea if you're planning on bringing a character with the unit, to keep him away from unwanted challenges.
Magical banners: If you're fielding a Standard Bearer already, you may as well get a cheap magical banner for him to carry. The Banner of Swiftness is probably my favourite as it allows you to control the battlefield even better with the extra movement. Banner of Courage is a cheap upgrade for those who're going to run their Boar Boyz on the flank away from the Inspiring Presence of your General. The Banner of Eternal Flame is only worth getting if you're adding a Big Boss or Warboss to the mob. Without it, the Boar Boyz still won't hit hard enough to kill of a monster such as the Hydra.
Big 'Uns: Unless you're running Gorbat Ironclaw, you're almost always better off getting an infantry mob of Big 'Uns instead. That being said, however, mounted Big 'Uns with will have an easier time hitting things while striking at a mighty S6 on the charge which is enough to even scare of some monsters. They aren't that much more expensive than their regular boyz, but the main drawback of bringing them is that it prevents your infantry mobs from making use of the Big 'Un upgrade and it's doubtful that the cavalry orcs will make better use of it.