Love the concept and artsyle. Takes a while to get used to, but that's to be expected.
I've only got up to 45 kills (curses, twin wave!), so I don't know if any new enemy types are introduced afterwards, but for the three that I've encountered (red skulls, twins and teleporters), I'd say they take good advantage of the 360 view, especially the twins and teleporters.
Took me a while to figure out how beat the former, even after I discovered the hint on the website (goes to show why reading before downloading is important), but afterwards killing them became natural (that sounds so wrong out of context).
One thing I think should definetely be added, however, is a kill sound effect. Sometimes it's hard to tell if I've killed a teleporter or if it simply telported.
Outside of that, maybe touching up the aiming angle, perhaps even adding aiming reticules, could help, but shooting works fine as is.
Overall, quite an impressive execution of an already impressive concept.