Hello, currently I am level 50 with almost full Rakata and I am specced lethality. I was mm a while ago but respeced.
Lethality:
In PvP (I am full Battlemaster) I love lethality going up against all targets except Assassins with their cleanse. Most all of Lethality's damage is internal which is great compared to mm, where my ambush can be shielded etc. Culls numbers are amazing along with crits from the Dots keeping my energy up fairly high, except when throwing in a OS into my rotation.
In PvE I feel that Lethality has slightly higher mobility than mm, although FT and Snipe and instant. Once again most damage is internal, and you do not have to be in cover to use any of your main abilities except SoS which is my favorite filler.
Cons in PvP: I find myself saving my main abilities until I find a target that has full or almost full hp. I do not use my Leth CDs on people who are close to death because a grasshopper might be close behind and require me to bring my dps to bear. Lethality also can not switch targets effectively due to its rotation.
Cons in PvE: Overall I love Lethality in Pvp, marksmanship will pull slightly ahead when killing multiple targets is required. Also when a boss goes into a phase that requires you to add something to your rotation (ex. Gharj Dr. Lorick) when adds come out or movement is required lethality runs into energy problems when a simple OS is added.
Sorry for the bad explanation, was writing in a hurry before raids start. Lethality does put me ahead than the other sniper who is MM and fight for top dps against our Marauder. Overall once you get nice gear (high crit and surge gear) you will put out a lot more damage and run into less energy problems because more crits= more energy.