I have read lot's of people saying it is way over the odds, IMHO all the transfer amounts are obscene in football today.
But you have to look at it this way if he is (and I would have picked him before he even came to play for us) Englands top/regular right back then you have to consider his worth in the same vein as Rio Firdinand, John Terry and even more comparable Ashley Cole. I am not saying GJ is as good a player as the others but he is Englands number 1 right back, like JT and RF are englands number 1 center back pairing and AC is englands number 1 left back and if they were to go on the market today while still under a two or three year contract they would all cost more than £17million so in that respect it is about right. Unfortunately really decent English players are few an therefore they cost a premium.
I think it is a very good signing and will prove to be money well spent.