Every time I've seen Sissoko for Newcastle, he was playing on the wing.(seen maybe 4-5 games)
Did ok the as I recall, but very on/off. So based on that, not worth more than 10-15 mill and definitely NOT what we need.
BUT: Then the euros start, Sissoko starts every game for one of the favourites. He is now deployed centrally, and seems an amazing aggressive, speedy, direct player. Wasn't even sure it was the same Sissoko. Definitely offers something different than what we have. Still not worth 30 mill, but that's the going rate.
To sum it up: He is probably twice as good as Ryan Mason(14.5), so I say go for it. Could be a massive success or a massive failure.