The SLR Magic 8mm is clearly not a perfect lens, but if you're willing to shoot at f5.6 or f8 and you value a diminutive ultra wide angle lens - The SLR Magic can produce surprisingly good shots.
Focusing can be challenging with this lens since it is extremely wide angle. It's critical that you focus at F4 (stop down later) and carefully check the center and the edges. I found that this took a little getting use to in order to get good results.
For pixel peepers the edges are a little soft. But in real world situations, I find the lens performs well at F5.6 and F8. And with a little photoshop cleanup it can produce impressive images.