I like the signing in large part because I believe in Poch and Lloris. There's no question that Sissoko has had motivation issues in the past, but this is the first time he's played for a "big" club in major competitions and he's always played massively when the attention has been on him (i.e - on TV, against big clubs, for France). I don't think we would sign him if Poch and Lloris weren't both on board with the deal.
Given that - he's a huge talent and he fits what we need badly. He's a fucking ox on the ball, he dribbles people like no other CM other than Dembele and he's pacey with a ton of stamina. He's got a nice end product as well, setting others up for scoring chances regularly.
If it wasn't for the motivation questions, Sissoko is a 50m midfielder in this current market. I have faith in Poch and Lloris sorting him out, and if that's the case I think this will end up looking like a steal.