He's a pretty high skill cap champion that has a lot of uses. However, he is extremely team comp reliant. He should only be played in skirmish / dive comps where he's not alone when he jumps. If you do end up playing him in a composition where your teammates cannot support you, he's almost purely a 1v1 or 1v2 duelist. This is speaking to assassin Rengar, though. He can be built in a variety of ways. In the Korean meta, he's played as a resilient skirmisher with usually only one or two damage items. I will say that you could even play him full tank and still be extremely useful to your team, as seen here. I can also attest that you don't have to follow the NA meta to do well (full AD assassin).
The biggest thing that other players have said is that Rengar is scary when ahead, and useless when he is not. My advice on this matter is (when following the NA meta): if you're not ahead (i.e. first blood and another kill by about 8m), grab a defensive item. If they have a double/triple AP comp, Locket is an extremely good second item for your team. Spirit Visage is better for your kit, if you'd rather go that route. Randuin's is best against the standard composition. A standard build by 20m would something like: Warrior - Boots - Randuin - Tiamat
This way, it's harder for you to fall behind and be useless. You'll deal more damage through simply surviving, since at this point, you can't 100-0 someone. This is also a good idea if you're already behind and/or the enemy team is fed.
With that, is Rengar good? In the current meta on the NA server, I stand by what I said earlier. He's very team composition reliant, and he will not do well by himself. I usually find myself playing Rengar when my team needs a way to create picks in the early game, as well as be able to delete a hypercarry. A composition that would be great with Rengar would be: Gnar, Orianna, Alistar, Kalista, Rengar and a team that I would want to play against with Rengar is most likely something like this: Hecarim, LeBlanc, Nami, Graves, Volibear
Compositions like this lack a hard peel to a Rengar once he has already jumped, and there is nothing stopping you from deleting LeBlanc or Graves. This is the ideal composition for Rengar, since his job is done easily and his teammates all have the ability or mobility to follow up on his engage. In these cases, Rengar is good.
I've been maining jungle since patch 4.20, played over a thousand Rengar games, and I'm Gold II. I hope you choose Rengar!