Yeah you'll be surprised what can happen with your ult when two cannon balls hit their mark. I once double killed two champs at 500 hp that were close together and chasing down an ally in a different lane. With a little map awareness, you can turn ganks against your team into 3 kills for your team.
I still highly recommend infinity edge after sheen, bloodthirster only boosts your attack, not crit chance and crit damage. Also I save on finishing the sorc boots until I see the enemy getting more magic resist. Haunting guise is usually enough to start.
My rune page is only partially complete, but right now it contains all crit chance marks and quintessences, with all magic resist @ 18 for glyphs. I will probably either make the seals either MR@18 or dodge, if I can afford the IP someday.