Seconding Composer on the level 3 feature.
Assassin's strike increases DPR by .5 per die of Sneak Attack damage, but that jumps to 2.5 per die at level 17. It also REALLY raises the floor of your damage-when you get it, you go from (with a Shortsword) dealing 21 (6-36) plus stat mod damage to dealing 23.5 (16-36) plus stat mod damage. I'd say that, as it is at level 9, it's okay. But the level 17 bump is NOT. That's +60 damage per successful sneak attack, which is pretty ridiculous.
All the poison use stuff is kinda situational, though-I'd replace it with something else. (Also, the ability to get +10 to save DCs... That's insane.)
Masterful Infiltrator is also crazy. That's 51 minutes of Invisibility per day at the level you get it, or in other words, invisible for every fight you have. Even if your fights last 10 rounds, you'd still need 51 fights to use it all up. You could easily spend half an hour invisible for something and STILL have enough left over to be invisible for every fight.