Probably one of the most useful out of all the midfield special abilities (the rest are crap)... As for usefulness, they are pretty useful... I've had shadow strikers score against me in the past so I'd go for it.

I currently have an MC/AMC player with Shadow striker ability, I'll let you know how that goes.

The player will score from long range strikes more often.. But less effective if the opponent has a defensive wall player, which of course makes sense.