Varus needs a good cs to be useful. try getting last hit on minions and if your support is farming tell them to back off. Make sure you are out farming the other ADC/APC on the other team. if you guys are getting poked at really hard make sure you call for lots of ganks. The best support you would want for varus is taric. Taric uses his stun to completely stop the enemy for 2 seconds, this is very useless for you since you do burst damage.

Varus is a champion that requires teamwork. Without a good support he is almost useless if not played right. Make sure you keep an eye out for there jungle since varus doesn't have any escape spells. Make sure your support buys wards to light up the jungle.

PROS
Varus does burst damage.
Varus is really good late game
Good early game if played well
Farms easily because of Blighted quiver

CONS
Needs farm to be useful
Varus often needs a good support
Requires team skills
Takes time to learn