>>28743592
It ranges from really good to really bad, generally depending on the design team and what block(s) came before it.
For instance, Theros so far has been a very shitty block, fluff-wise, with a few exceptions. Cards not matching their flavour, references to ancient Greece ham-handedly shoved wherever they can, etc. But it's come at the end of a long line of decently fluffy blocks (Scars of Mirrodin went crazy detailed on the Phyrexian factions, Innistrad is probably the most flavourful block in modern bar Lorwynn/Shadowmoor, and RtR comes from a very solid foundation and concept), so it's understandable they'd be burnt out by now.
Similar thing sort of happened with Kamigawa, another top down design set that came after both Mirrodin and the Otarian storylines, which both in turn came on the heels of the Apocalypse.
At least, this is all in my opinion, and I'm probably biased against Theros for other reasons, but yeah. There's some really nice stuff in this game if you look for it.