Should I get shadow maul on my DM/SD?

I have never leveled a DM character passed level 10 so I have no idea how to choose powers that will be effective at DPS in the higher levels. Can anyone that is experienced with the dark melee power set give me some advice?

At very high levels with IO'd builds you won't use Shadow Maul in your single-target attack chain. It still remains useful for groups of enemies, and is a staple while leveling.

Most scrappers will eventually face large sized mobs. Many (if not most) scrappers plan on soloing x8 missions at higher levels. Under such conditions, AoEs are ideal. Hitting multiple targets takes virtually no effort in full size spawns. Considering Dark Melee has not other "true" AoE attack, I strongly recommend it.

If you prefer not to fight large spawns, you could argue against Shadow Maul,

I'd say try the power out, if you like it use it.

I personally don't like it and didn't have room in my dm/sd build to slot and get something out of it so I personally skipped it. It is one of the few aoe powers you will get from that combo so it really comes down to what you want imo.

Shadow Maul is also capable of holding whatever PBAoE IO set you might need/want later. Mine currently has a full set of Obliterations. Damage Bonus, Accuracy Bonus, Recharge Time and Melee Defense make it an easy set to want to get in as many times as you can.

To see which powers are best for single-target damage, look in Mids'. Go to Window, then Powerset Comparison. Select your desired powerset. On the left side, click on Damage Per Activation Time, or DPA. This is the best reflection of how much DPS a power has the potential to do without procs or enhancements.

Like most AoE powers, Shadow Maul has too long of an animation time to compete with single-target powers if you want a top-DPS attack chain. Most DM/SD builds depend on their ancillary or epic pool to gain decent AoE attacks.

I don't miss it, but I also have Sands of Mu.

I have been playing DM toons since the game came out and I have tinkered over the years with A LOT of Shadow Maul variations.

So, my 2 cents, feeling like I have the experience to comment on it: To really get the use out of it, you have to play with a style that lines up the baddies in the range and in the content I'm playing now, at high levels, I pretty much don't use it for anything other than a bonus set holder.
Even as a toon starving for a good "AOE", I just find myself using it less and less (there is something about starting that animation when I see "Maurauder activating Nova Fist" that makes me cringe).

Overall, the game seems to play a lot faster now, especially in teams and with Incarnate abilities and I just don't find it very useful. I do have it still on about half of my DM toons, but its for set bonuses almost exclusively and gets occasional use. And like SANC said above, if you have Shadow Maul, you really don't feel like you are missing much.