Sorry but Essien, Diarra lot comes about every 3-4years. But Cassano's sheer talent is one that doest as quite as often - I will certainly put full well behaving Cassano in front of those two anyday of the week and twice on sundays. Ditto for Robinho, Christano Ronaldo, Messi, Fabregas.
Lets say I am not a fan of dolling out millions for a defensive midfielders. It does not quite reflect the general abundance of those types and relative scarcity of "fantasistas". Take away Essien and Diarra, and stick in Sulley Muntari instead - I think the result would be quite similar.
Anyway, I haven't seen Cassano for a long time (for obvious reasons) but I always felt his biggest deficiency is that he is not that good passer of the ball - especially taking into consideration he competes with players who are technically excellent. That means he has to primarily operate down on the left hand flank and try to use his trickery to maximent effect. The problem is then that he isnt quite Henriesque quick enough to cover the same range of pitch and pose similar threat.
Only way I can see him elevating his game is that he has to become more "box" type and become 16+ goal scorer instead 9-10. Who does this remind you of? Yes, the one and only Del Piero. I think Alex can become an excellent tutor for this lad.