so far the high profile ones come to mind, given the expectations:
Berbatov - Manchester United - 30M
Keane - Liverpool - 20M
Deco - Chelsea - 8M
Robinho - Manchester City - 32M
Berbatov has been described as languid, lazy and disinterested. Man U fans seem to have had enough and are calling for blood.
Keane couldn't even get a pigeon sweat for Liverpool which begs the question, why was he bought in the first place? The IOSS episode is funny but true.
Deco has struggled with injury, and when he's been fit seems out of place amidst Lampard and Ballack. Definitely falls below expectations. Not as expensive as the others, and considered a bargain - but no production means low grade.
Robinho: for the most expensive player, you expect magic on the field. Hot and cold performances all season long, and not able to dominate defences as he was expected to.