Mahou Shoujo Site Sept serves as a spin-off of the Mahou Shoujo Site series. Therefore, reading it is not crucial to the main story.
The art was pretty average, not better or worse than in the series. What I did like were new magical weapons and abilities. Weak sides of this manga are mostly characters which lack development and quite predictable story. Some of the characters appear also in Mahou Shoujo Site, but that it not interfering with the story.
Overal for me 6/10 since I didn´t enjoy this story as much the other works from this author. However, the manga is quite short, so you should give it a try.