The Descent of Angels is an underestimated and powerful rule. In my opinion anything that so radically allows you to alter a fundamental game rule is extremely powerful. I am not so enamored with the ability to scatter d6 less inches since that is rule that already circumvented in other codices with teleport homers and marks or icon of chaos. But with that said it still can’t be ignored as an advantage considering it costs zero points.
It is much easier to thread the needle for important positioning when you only have to consider the possibility of scattering 6 inches. You will most always land within the critical melta rule range. Remember you place you figures the way you want them after you land. Your melta gunners should always be as close as they can be to your primary target.
The real benefit to the Descent of Angels rule is the ability to re-roll a failed deep strike from reserves roll. This radically increases your chance of coming in with your entire force on turn 2. It raises the units 50% chance from arriving on turn 2 to 75%. This is huge and a complete game changer if you like using the deep strike rule with your assault troops. There is much less fear of coming in piecemeal. This allows you to saturate your enemies fields of fire making them make a lot of tough choices as to what they should shoot where as before you may have only had half your force on the table. With a little luck on turn two you are dumping your whole army (complete with super characters like The Sanguinor, Dante, and Lemartes) right in their face on the 2nd turn.
This puts a great deal of pressure on most opponents. They may find themselves completely out of position or too spread out because you started with nothing on the table in round one. This tactic will give all but the best generals fits if you are smart where you decide to put your boots on the ground.