Personally I found I immensely disliked MM during beta and pre-patch. However, yesterday I got my Thas'dorah and somehow... it worked. Maining MM right now. Here is how I roll it, hopefully it can help you out:
Build:
Lvl 15 tier: Lone Wolf in party / Steady Focus + pet when solo
Lvl 30: Lock'n'Load in party / Black Arrow when solo
Lvl 45: Posthaste
Lvl 60: Patient Sniper
Lvl 75: Binding Shot
Lvl 90: A Murder of Crows
Lvl 100: Piercing Shot
A lot of builds out there are focused around Sidewinders + Patient Sniper + Lone Wolf/Steady Focus, but I found myself hating the talent with passion. It gave me a lot of down time between Sidewinders' cooldowns, despite great damage output, making the spec feel boring and clanky and altogether un-enjoyable for me.
Next thing I tried was to drop Sidewinders and Patient Sniper and instead go for Sentinel + Piercing Shot. It wasn't bad, in cases of bad RNG I could always pop a charge of Sentinel to refresh marks + it worked great in AoE to build up stacks of Vulnerable, but... in the end, it also didn't work out for me, although in this case I cannot put my finger on why, unlike Sidewinders'.
On a whim I tried out the build above and it just... fits for me.
Single target (with pet):
Opening with Piercing Shot, followed by AMoC and Windburst. Triple Arcane Shot to trigger Steady Focus. From there depending on RNG. Marked Shot whenever it procs, after it makes target Vulnerable at least one Aimed Shot, with enough Focus 2 in row, it has just enough time for both to be buffed with Deadeye, then back to Arcane Shots.
Due to Marks' pet being so... squishy, I usually hold Black Arrow until it's needed to take aggro off of my pet and continue ping-ponging the mob between Dark Minion and the pet.
AoE:
Drop Binding Shot between myself and group of mobs, use Piercing Shot on furthest target. Pop Trueshot if available and follow though routine of Multi-shot -> Marked Shot spamming. Loot
That's pretty much it. I found party gameplay with Lock'n'Load to be more enjoyable than with Black Arrow, admittedly. If the pet wasn't so squishy even in Tenacity, it would have been interesting to try out Lock'n'Load solo with pet out. But alas Black Arrow is very needed to keep the pet alive. Hope it works out for you
On all specs I also dragged out Mend Pet and stuffed it alongside damaging abilities to use it on CD more easily (or to not have an OH SHI--- moment when pet dies and I'm fumbling around in Pet Utility in panic, as Mend Pet automatically switches to Revive Pet), but that's neither here nor there
Edit: Next artifact trait for me, Mark of the Windrunner, will make Windburst apply Dead Eye as well, which will slightly change and smoothen out the rotation. Now I only need to actually get to it with the Artifact Power crumbs...
Maybe it's just the playstyle I'm not used to, but I feel like paper and my pet is even worse (I know ideally they shouldn't run with a pet but I can't give mine up).
