While typing this the one word that kept coming to mind was ... dull.
If you have $9.99 to spare and will do anything to have a Warhammer title no matter the quality then, uhm, ... ok i guess so.
WH 40k:SoV is a repetitive un-inspired Warhammer themed game and at best an average lane defence game. It has bland sound effects, and the graphics are dull with neither using to any potential the richness that could be put in considering the source material around which it is themed. Ignoring the WH 40K theming the graphics are still, especially the backgrounds , dull and the sounds bland.
Over all the game is not as much fun as some free to download TD or LD games. The usage of Warhammer 40K imagery is irrelevant as it ignores the mythos created around Space Marines and treats them as literal cannon fodder (or like Imperial Guardsmen for the WH 40k fan) to be wasted in useless lane combat.
I would say at best i would give this game 4/10 and that is because I am a fan of Warhammer 40k and so just skinning it to represent the noble and glorious Adeptus Astartes gives it a few points.
I must add an addendum now though. After a few more hours of play I have to say that it is mildly addictive and will eat up some time initially. I reckon I may play an hour once every month or two till i bore of it for a bit again.