I don't care for the black covers - I don't actually dislike them, and I have no objection to the idea of doing 'classy' covers to attract readers who might mistake Discworld for children's books or 'zany' and 'wacky' based on the Josh Kirby covers. I just think that the photographic covers lack something.
I am still enormously fond of the Kirby covers - they're so iconic of the DW now. And I love Kidby's covers too, he really is a marvel for getting the characters right.
But I can see the need for something that doesn't look so genre-bound. Kidby's work isn't cartoony like Kirby's but just by dint of being an illustration on the front of a novel it immediately positions the books as 'fantasy' or 'for kids' and it's a shame that readers might miss out because they think that the series is just for those people. There's been a move in recent years to present 'serious' fantasy in less genre-fied covers (e.g. George R.R. Martin's Song Of Ice And Fire) and I do think Discworld could benefit from something like this in addition to our beloved Kirby/Kidby editions.
As an illustrator and designer, suffice to say I've never put much stock in 'never judge a book by its cover'