as far as i'm concerned the jury is still out on this one. personally, i think it looks like someone added wings to a vacuum cleaner, but blocky is the style for space marines.
now if only they could perfect their pricing...
+edit/update+
thanks to xenite for his link to this image:
which correctly identifies the aerodynamic inspiration for pretty much all space marine vehicles. of course, any suitably inebriated rocket scientist will inform you that even a brick will fly provided you strap enough engines to it...
all images property of games workshop except for that last one which is all xenite, baby!