i rarely ever see him in game, however when i do see him, he usually is a big game changer, in close range fights he can tear down entire teams, his ult can either nuke large areas or close off escape routes
he can be out ranged but if you get in range, its over
i have never personally played him myself, so, like vayne, he may be really hard to achiee these results with, but yes he is a really good champ, in the right hands, but who isnt?
and about the passive, its actually more useful than you would think
get into the "hot zone" or whatever its called, then nuke them with your extra damage spells, now you cant cast for a few seconds but your auto attacks tear through their low hp pools, this is enough to make any team focus you hard, good thing your basically a tank
i always say try before you buy, but he is on sale, take a gamble
worst case is you have a new champion to stomp bots with lol
that happened to me when i bought corki and played horribly with him