Here's my updated version. I took most of the feedback, and tried to add it in. I let the tentacles grow a bit, so they're filling up more of the scene. I adjusted the hue/saturation, and tried to match more of the overall color of the background... and the other big change was the addition of the guy running away. If you notice, one of the tenacles has touched him, and transformed him into the same chrome material as the monster (or whatever it is).

I was experimenting with more atmosphere, like Justin suggested, but I ended up not liking the result. Overall, I'm happier with this revision, as it seems to be much more interesting now. I do agree that the background scene could be a little more interesting, but I'll stick with this one for now, as I have a lot of other scenes for different pieces I want to do, so I'll just use this as a great learning experience. 

I love 3D and compositing, and have been learning a lot of X-Particles (Cinema 4D), lately. My original plans were always to do this kind of work with video, but it was proving to be too difficult with rendering times, and planning the shots with the proper equipment, collaboration, etc... so I decided to take it a step back and focus on doing composites with still images. 

Here is one of my early attempts with a photograph I shot on my own. I used an HDRi for the reflections/lighting, and did a small amount of matching the whites/blacks with the van you can see it the background, and its tires. It was an overcast day, so I'm not entire sure if the shadow it very accurate, but it looks like it fits in.

Would appreciate any feedback/criticism.

Thanks!!

-Brant