Q: Scales greatly with AD, when you are ahead max it and it makes you further ahead with the AD you should be buying! (Make your enemy in lane hang onto the tower for dear life!) *Charging it when your kill-lane partner is going in usually leads to a sure hit, as they normally have a stun*
W: Passive damage for last-hitting (if you are bad at that sort of thing), max it first if you start falling behind and want to try and trade. (% life damage may be "bad scale-wise" early game, but if you lack AD, it may be better in direct trades to have a bit more damage, both % after using a skill and the base on-hit.)
E: AOE slow with half-healing, max it if you want more upfront burst damage or slightly better chasing/fleeing capabilities!
R: The "you-are-f**ked-now" abillity... "And you, and you, and you and you!"
Next week on underused adcs: Sivir, she has to be a counter-pick but she can be good! Find out how!
(Btw Riot, some people like playing Varus AP, I hate that, but I don't wish for it to be a fotm and for him to be nerfed in some way. Some people have SOME AP on Varus via blast due to getting the Magic Pen %.)