Thank you... fan bois alert *
Yeah GW does some stuff very well indeed, it also does some stuff rather poorly but this is a matter of taste.
Mantic doesn't have a class writer like Bill King writing easy reading background for them, but then it doesn't have a total load of rubbish coming out from Gav Thorpe et al either.
I don't know the Mantic background at all, (do they have any?) but then I don't care about that stuff, I don't need that from a miniature manufacturer. I make my own background where applicable and get my background from a wide variety of authors, I don't like mine spoon fed to me in a format aimed at 12-15 year olds. But hey others do and thats cool too.
GW have made very little of their own fluff, it was almost entirely plagiarised from other well known authors, whether in fantasy or 40K.
I don't have a problem with that at all it just makes me laugh when people think that GW came up with 'all' the ideas and fluff on their own.
Where GW have been smart is too copyright a lot of words and concepts to stop others from using them.