Rengar is one of the most bursty champs in the game. His current state is like late game brand where there is almost no counterplay if you have your items. Rengar offers great roam ability at 6 even though you are a top laner. As Rengar I recommend staying in your lane until your Ult is up then roaming over and over again until you and your whole team is fed, which happens almost 100% of the time as long as you do it right and don't feed.

So basically how I farm is jumping from the brush onto the minion with your Q or just auto. This way you avoid fights with the laner without full furocity. Once you have full furocity and are level 2 I recommend taking w and all in the laner with a Q-Q-W combo which basically wrecks them.

So basically what you want to do is have full furocity and jump on the person with your R then Q-Q-mash all buttons to get a kill. Rengar is not the hardest champion to play but it does pay off if you do have him in your champ pool.

Pros:

Rengar offers so much burst that you can instantly kill anyone out of position and is key in teamfights to take out the carry.

Cons:

After rengar has used his full combo there is about 4-5 seconds where he doesn't do anything but auto, which leaves him very vulnerable to be killed. So once his combo is over he is useless until his spells are back up.

In my opinion Rengar is a great pick for either top lane or jungle (top preferred). He offers great burst and late game carry potential. He does have some good counterpicks but overall as long as you roam you will get fed (in most cases).