I wrote a similar thread in the past but I have to change the details after more testing and comparison with spike traps. So here is the updated information from my tests. Feel free to correct me if you find anything that is not accurate.

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Cluster Arrow
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http://us.battle.net/d3/en/class/demon-hunter/active/cluster-arrow
50 Hatred
225% Fire damage within 6 yards on impact
100% Fire damage within 3 yards randomly “bounced” grenades (x4)
The grenade will not explode if no enemy triggers it when it land = 0% damage

To optimize this attack you want to corner up the mobs, put them into a corner or create a natural cluster of mobs so the grenades will not fly everywhere but landed very close to your center of impact. The potential maximum of this attack is 625% weapon damage but don’t forget that the grenades have a delay after the impact, which takes about 1 second.

The one rune that seems to not follow the standard is Cluster Bombs. The cluster launches into the air and follows a slow moving trajectory, dropping grenades as it flies. 5 grenades are dropped and each deals 230% weapon damage to all enemies within 6 yards. Unlike the other runes, the range of this attack is fixed. Start dropping from 12 yards in front of the demon hunter and travels 35 yards ahead. Most targets sitting at 35 yards away from you will take 690-920% weapon damage depending on the size of the monster.

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Enough about mechanics, I want to see some difference in each rune and at the same time increase their potential. My list of suggestion will not satisfy everyone.

Dazzling Arrow
Instead of releasing grenades, the cluster releases a lightning shockwave that deals 55% weapon damage and stun all affected enemies within 14 yards. Reduce Hatred cost to 40 and turn Cluster Arrow into Lightning damage.
- same 6 yards impact aoe
- does not release grenades, instead release lightning that paralyze
- affected enemies are stunned for 3 seconds.

Shooting Star (instead of Shooting Stars)
Shoot off a cluster of rockets that penetrates enemy’s defense and explodes on impact for 525% weapon damage as Fire.
- same 6 yards impact aoe
- does not release grenades
- can penetrate Waller and Shielding abilities

Maelstrom
Instead of releasing grenades, the cluster creates a whirlpool of shadow energy that deals 330% weapon damage over 3 seconds to all enemies within 6 yards and immobilizes their movement for 3 seconds. Gain 3% of all damage done as Life and turn Cluster Arrow into weapon damage.
- same 6 yards impact aoe
- does not release grenades, instead release shadow energy that lock and drain life
- affected enemies are rooted for 3 seconds.

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Spike Trap
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http://us.battle.net/d3/en/class/demon-hunter/active/spike-trap
30 Hatred
275% weapon damage within 8 yards once triggered
1.2 seconds arm delay
The trap will explode when an enemy stop by its edge = 0% aoe

To optimize this attack you want to place the trap directly in-front or underneath your enemies. The trap will not trigger the monster until damage is made. So if you can place as many as you can before the chain reaction starts you might be able to finish the battle before anything else starts to move. If your aps is lower than 1.88, I recommend using Long Fuse to obtain the same result.

If you like to move and single out mobs I recommend using Lightning Rod. It is not the highest damage dealer, but it is the widest spike trap on the list. Once triggered it release chain lightning that hits up to 3 enemies within 20 yards of each other. It does not have an explosion aoe so it makes no difference where you place the trap along the path of your enemies.

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Another one of my annoying alternative to the skill runes...

Sticky Trap
Plant the trap on an enemy. After 30 seconds the trap explodes dealing 165% weapon damage to all enemies within 16 yards. If the target dies within 30 seconds, the bomb explodes dealing 440% weapon damage to all enemies within 16 yards.
- doubled aoe, 16 yards
- 60% more damage if triggered by death

Grand Fusion (instead of Long Fuse)
Link and place all 3 traps as if placing a larger spike trap. Increase the arm delay to 2 seconds and increase the damage to 550% weapon damage to all enemies within 24 yard.
- tripled aoe, 24 yards
- 100% more damage when triggered

Lightning Rod
When the trap is triggered it releases a pulse of lightning that strikes up to 3 enemies within 20 yards of each other for 275% weapon damage as Lightning and has a 35% chance to Stun each affected enemy for 2 seconds.
- same 8 yards aoe
- release lightning that paralyze and stun for 2 seconds.

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When I started the Game I was told to use Spike Trap or Cluster Arrow, but not both. I’m now playing with both skills slotted just because I’m testing their effects. My conclusion is now these two are the best together when you put them into work.

Lay trap before battle and end it will multiple cluster arrows.

If you think you read some of these suggestion in the past... It might just be me again. I did change the wording but the mechanics I'm adding are the same...