5 million is still quite a lot for a player in the last year of his contract, even for one so talented. It has been reported in recent months that Duff lost a yard or two of pace because of his injuries and that this has made him less effective, I would have to say there must be something to that, if not I can't see why he would have been let go. I know that Chelsea can buy anyone and have a very strong midfield, but players as versatile and skilled as Duff are hard to come by, so there must be a reason why they no longer feel he is up to it.
It could turn out to be a very good signing, but had I been the Newcastle manager I would have looked to use the money elsewhere. Even if he was "only" 5 million (I can't see Newcastle having uch more than 10 million to spend this summer so they have used half of their budget on him), he will have eaten up a sizeable portion of their wage packet.
I don't wish him all the best. He is at a rival club, he hasn't been a Rovers player for years and if he pulls his hamstring tomorrow I would in many ways be happy.