[emKay]

This is a bluff very often. With you flush draw and the overs, you have already near to 50% equity, add the fold equity and this is a standard shove. With the 55, Im torn between shoving the flop and calling and calling/shoving the turn on a non-spade.
I expect him to bluffshove the turn a fair amount of the time, plus he likely shoves draws that didn't come in and any pair. The only problem is you cannot really call a turn shove when a spade comes, allowing him to bluff you off the best hand.