Quintessence of attack speed : Is good because in S7 is nerfed rengar´s Attack speed.

Mark of Lethality : Good to killing tanks. In S7 is very strong tanks, than you need some Lethality.

Jungling: If you kill 4. position in jugnle. You go gank on link what is worst. Than you go on Blue buff. And go back on spawn to take Smite. And than go gank next link.

Pros
+Very good speed (Boots of Mobility)
+Very srong
+Good ganks
+Very good help to team
+Fast in Jungle Farming
+Huge burst potential on isolated targets


Cons

-Pink wards will make your escape hard if you run with Ult
-You are almost as squishy as the targets you want to get to

Not go 1 vs 3 or more because don´t forgot rengar is not tank. Wait on your team tank and than jump with your Tank into all enemyes. If you not have a tank the team fight starts support. Support attack on 1 of enemyes and you jump on him with R than your team have info ''We can fight'' Than you have teamfight 5 vs 4 because you kill 1 with your R jump.