Rend is great right now, to put it bluntly. A weapon profile that includes rend is dramatically more effective at putting out damage compared to non-rending attacks in the general scope of things. There may eventually be some special unit rule or Realm of Battle scenario/battleplan that makes some changes to this, but for the moment you can be confident in saying more rending is better. Typically, you won;t have rending in most of your army, so you'll need to find the way to best use the rends that you do have. Typically, you'll do just fine cleaving anything your renders get tangled up with, but rending is most effective in certain situations. The shorthand version is that rending is better against units with better armor saves.

Below is a chart outlining this effect - assuming you have successfully dealt 6 wounds to be saved by your opponent, the number of total wounds suffered after armor save is listed and then the result applied based on increasing rend amounts.

The take away message from this table obviously is that higher rend is more effective, and becomes more useful as it targets tougher opponents, and more redundant as it hits lightly armored foes. You are only going to have a limited number of rending attacks, so it's better to spend them targeting the hardest mothers your opponent brings to the table. Why waste the massive rend your dragon has on some blob of 6+ armored guys, when the overkill 'wastes' its rending power? Applying rend to a 6+ save unit increases the number of wounds you deal by 16%, which isn't paltry, but consider this: A rend of 1 against a 3+ save will increase your chance of wounding by 50% (2/6 wounds vs 3/6 wounds). A rend of 2 on the 6+ unit still only increases your wounds by 16%, but against that same 3+ save unit, you now have doubled your wounds dealt from 2 to 4 out of 6, for an increase of 100%!

Nothing in the game so far sports a save better than 3+, but can be improved thanks to the effects of terrain or mystic shield. A unit that had those benefits would become difficult to damage, if not outright un-killable were it not for the effect of rending. The next time your Duardin opponent laughs as he bunkers down his heavily armored elite unit of choice, make sure to toss your best rending units his way and wipe the grin off of his face. Even if you're fighting against the bare chested chaos hordes with little to no armor, rend can completely remove the save from some units, demoralizing your opponent if nothing else. However, if you have the choice, you would be best served funneling your rend toward armor, and leaving the lighter armored troops to clash with your rank and file.