From the bits I've seen of him, he looks OK. He has got skill and pace, can cross, shoot, and is good at dribbling and keeping hold of the ball, which makes him a good outlet.
At £4m, I really don't think that he is too big a gamble, and (hopefully) leaves us with more to spend elsewhere.
He will provide us with some natural width, and as someone pointed out in another thread before, maybe Martinez sees Pienaar coming inside more.
From the players that have thrived under Martinez at Wigan, NZogbia, McManaman, etc, it's clear he likes fast, tricky wingers, and with him on 1 side, and Mirallas on the other, I honestly can't see the problem.
Maybe if he was called Santos McGeady Silva there wouldn't be as much of an issue