I don't understand why so many people dislike Hamsterley...
It's all relatively subjective, of course, but nothing really happens at Hamsterley. The DH trails are decent enough, but the "red" route is almost devoid of features. It's like you ride out, see a load of fire roads, a little bit of shoddy singletrack, some more fire road, then back along a dull little groomed trail to where you started. Quite how it got a Red grade is beyond me. The whole place just feels like tiny bits of singletrack joined up by tedious fire roads. There are one or two good sections, but nothing, in my opinion, worth the drive from Newcastle.
Kielder at least has some interesting singletrack. Even with its huge fire road climb, the singletrack on the new Lonesome Pine route is more interesting and more challenging than anything at Hamsterley. Quite what it'll be like come the end of the year is another matter; they've ran out of money and there's (virtually) nobody around to repair the trails any more. Deadwater is already starting to look rough around the edges and we're not even into the summer season.
Like I said, what people like about trails comes down to what people like about riding. I can see the appeal of Hamsterley for people who just like a nice, contained pootle around and enjoy being outdoors on a bike - the dirty roadie types; but in terms of a technical challenge I don't think it offers much at all.