This lens really has some incredible properties.
Here's a few of my fav shots so far. These are all right off the camera (and uploaded to FB).
So, there's the good. The lens is really pretty amazing. The lens obviously distorts everything so it's hard to decide how sharp it truly is. I think sharp enough for the type of pictures you'll be taking with it. As for DoF, at 4.5mm it's almost impossible to tell the difference between f2.8 and f8.
My partner says it's like being in the glass globes from the movie Labyrinth, and if she's shooting, this is the second most likely for her to use.
Here's the bad:
Your feet will be in the picture. Having the full 180 field of view is fun and bizarre, but it means that you have to be super aware of where everyone, including yourself are. I've gotten my fingers in the picture just from holding onto the lens, and as for your feet, it's almost guaranteed to capture them.
This lens isn't reporting focal length to the K-7 body, or the K-7 body isn't accepting the focal length, and you cannot choose the focal length like you can with an A lens. This means that the SR on the k-7 goes absolutely NUTTY. It looks and feels like it's trying to correct an 800mm lens rather than a 4.5mm lens. If you turn on live view and wobble the camera, you can actually feel the SR jerk the camera about and watch the image move all over the screen.
If you do not use the work around of turning off SR, you simply cannot take a sharp or useful picture with this lens, even at 1/2000th you'll get double images.
Sigma support has been a little underwhelming on this issue, saying their k-7 with the lens attached does similar things, but that it shouldn't affect quality. When I sent them samples, they suggested I send in the lens to them for repair or have Amazon send me another one. Amazon has sent me another one, and it's got the same issues.
I'm still waiting to hear back from Pentax support. I'll ping them again on Monday about it.
If we have any Sigma or Pentax reps on the forums, I'd be happy to send you a link to the full res sample images demonstrating this problem.