Oh, his ult is amazing and saves so many asses and allows You to be anywhere with a team. Problem with Shen on top is that he will probably lose his tower first because he will be helping team with his ult, but top tower isnt that crucial for a win, so its not a big problem.
He's ult is useless in lanning to, but You have so much harass ability because of his Q that it doesnt matter.
I build him a little different then guys up there, for example skip randuin completely and do atma really rarely, i prefer to build hp on him because of Force of Nature passive and Q.
Dorian Shield [armor + 5hp pots against Lee Sin, magic res + hp pots against Rumble], mercury threads, sunfire cape, giant belt -> negatron cloak -> force of nature -> warmog.
There are only 4 champs that can sustain against Shen on top: Yorick, Lee Sin, Rumble and Riven.