He is miserable to play against if you are an ADC and they are a competent player. Every teamfight I would enter the battle as Quinn and just be repeatedly instagibbed. Even split pushing was dangerous because at any moment, Rengar could be sneaking up on me with his stealth ult if my whopping two wards weren't enough to spot him ahead of time.
Can we please get an ADC equivalent of hourglass? Seriously. As it currently stands, that is the sole item that givens me even a chance to survive against his onslaught and many AD caries can't even make use of the stats it provides. Yet despite that, Rengar is so frustrating and dangerous that even with the nerfs the item is worth the buy.