I'm alright with it. As they push on the main-series this is the fillers in between. I'm uncertain about Annandales writing skills as I've yet to read something spectacular from him yet.
Gav Thorpe handling the Lion after ADB or Nick Kyme handling the Night Haunter.I'm somewhat surprised that they have not done this before now, given the fact the primarchs are the beating heart of the 30k series.
My biggest concern though is the fact that as the primarchs have been fleshed out I find myself becoming more and more picky as to which author I like to see write about each of the Emperor's sons.
It is like watching a remake of a movie with an iconic character that has always been played by a specific actor and the new actor.... just doesn't get it right.