Tactical Suggestions:

Because of the need to keep the Dark Reapers still (the reaper launcher is move or fire) it is important to pick a good position with some cover during deployment that has a good field of fire. The relatively long range of the reaper launcher enables the Dark Reapers to be kept back behind the front lines of your army. . Don't put the Dark Reapers too far back though as you want to deal with any troops that might advance towards your lines. This is particularly important during the "night fight" scenario, as visibility can be reduced to quite a short range. Dark Reapers should be kept behind some other troops, to screen them, as they will attract a lot of fire. Dark Reapers will usually kill whatever they hit. However they are vulnerable to return fire and assault.

During deployment, try to place the Dark Reapers last during the heavy support placement. The reason why, is that that you want to see where your opponent has placed their barrage weapons before you commit to deploying the Dark Reapers. Barrage weapons can make a mockery of your plans, as a barrage can not only kill the Dark Reapers it can also pin them, rendering them useless for a turn.

If you can't find a good position in your deployment zone for your Dark Reapers, they can be transported in a Falcon (the best choice due to its low troop carrying capacity and versatility) or a Wave Serpent. A Falcon with Dark Reapers in it, is a lot of points in one place, so it is a good idea to screen them or move them around behind cover until they have disembarked for their better position.

The reaper launcher is very good at killing Space Marines in power armour and other troops with high toughness. The fairly low strength (for a heavy weapon) of the reaper launcher means that is only marginally useful against, the lightest armoured of vehicles.

To maximise the effectiveness of the Dark Reapers, it is recommended that squads of five are used. You want to kill as many of the enemy in a squad, with as few squads as possible, of your own.

Because of the effectiveness of the Dark Reapers, they are often made a target by infiltrating or deep striking troops. Of particular danger are assassins. A way to guard against assassins is to have some Howling Banshees or Striking Scorpions (Striking Scorpions are a better choice, because of their armour, as it makes them more durable to incoming fire) near by. Surround the Dark Reapers so that assassins (particularly the Callidus) can't assault them directly because of the rule that states, that troops can't be placed on the board less than 1" away from an enemy figure until the assault phase.

Placing an Exarch with a squad of Dark Reapers isn't that good a use of points.