He's a pretty down to earth guy, he took his chance at Ajax when Vermeer made too many howlers 6 months before the WC. He accepted being subbed off pretty quickly saying 'We're a team and we're doing this together'. He's an all-round keeper with the occasional howler himself, but he's been great ever since he's been first choice. He doesn't know fear or pressure in a game: I've seen him turn top strikers in the CL so many times when they tried to put pressure on him. In his pre-WC year he was voted Voetbal International player of the year, and last season he was Ajax player of the season so he indeed has come a long way in a short time.
I haven't seen every match of him to form a good judgement, but from what I've seen he's been good. For me that's not enough to earn a transfer here, but I'm sure van Gaal/Hoek will judge him better than I do. I can only see this transfer happening if there are no other available keepers out there who are good at the sweeping part though. I don't see it as a particular strenght from Cillessen but he's decent at it.
We could do a whole lot worse than Cillessen.