Reaper's Mark from Deathmark's Shadow Set, how strong? how to use?

I (Blademaster, cold+frostburn) finally got the final set part and now i have to decide to either use Ring of Steel (3pointed) or Reaper’s Mark on my RMB.
I know i could/should use both. But i will stick to one.
Furthermore i find it strange that it has no CD at all. And a rather long duration and -15%RR which is also rather high. Seams strong.

Whats your thoughts about this skill?
How and when do you use it?
Wouldn’t it be “better” if it would have a CD in regard to %proc Chance of various Devotion skills?