I personally love my MM sniper. I run with a "pure" MM spec 31/10/0 as it gives me the abilities i want/need from Engi and also gives me 90% of the stuff in MM. When it comes to PvE (my bread and butter if you will) MM is hard to beat. I can consistently do more dps than a full Engi specced sniper and a Lethality can't even come close. Ironically on my serer, Jung Ma, the most recent top dps was a MM specced sniper. As for PvP; I can't really say much. I don't generally PvP but the few times I have, I haven't had any real fun. MM tree has very little survivability rate outside of cover and it doesn't take long to have yourself targeted when you start critting for 4-7k. After that, be prepared to get targeted down and die quickly.
why on earth would you not take 3 points in Lethality in the Lethality tree?? 3% to your crit chance!!
That's 3% of crit rating that you don't need in your gear; so, you can stack more power!!!
p.s. Eng sniper for the win!!!!!!