I own a Peleng 8mm and have shot a ton with it. Back in the summer, on one of my trips to Yankee Stadium I rented 3 lenses I was interested in trying out for the trip.
1. Canon EF70-200mm f/4L IS USM
2. Sigma 8mm f/3.5 DG EX HSM
3. Tokina 11-16mm f/2.8
I had already been shooting with my Peleng for a couple months and wondered if it was time to do what I had originally planned to do... get the Peleng to see if I would use a FishEye enough to justify the more expensive Sigma. Both lenses have a learning curve, and I had all but mastered the Peleng at that point (one can NEVER have 100% mastery of any lens if you ask me, we are always learning), but I had found the lens's sweet spots and it became one of my favorite lenses. With the Peleng I'm able to get incredibly sharp - i.e. Zeiss sharp photos most of the time. So, I was wondering if now that I found I really liked shooting with the 8mm circular fisheye if in fact it was time to upgrade, so I rented the Siggy.
I started doing some tests with the lens around the house with the camera mounted on a tripod and the same subject matter being used to test the two lenses side-by-side.
Here's what I found...
a) The Peleng is much sharper in the center of the frame even @ f/4 and @ f/8-f/11 it wins for 95% of the frame... the area it loses to the Sigma is at the very outside of the imaging circle out by the vignetting areas. The reason for this is kind of simple, the Peleng has an adaptor, the Sigma has a native EF mount. I even swapped mounts on my Peleng (between the EOS mount - which sucks and I hate, and the M42 mount which I screw into a M42-EOS adaptor ring). This actually showed me something I had overlooked in the past, and that is that the EOS mount, which is very poorly machined actually registers the lens a bit closer so the perspective is a slight hair different, just a bit wider - but nothing worth writing home about.
b) The Sigma is sharper near the edge of the vignettes, but overall sharpness is lower.
c) The Sigma has better flare control due to more modern lens coatings. All these lenses will flare, but the Sigma controls it a bit better than the Peleng.
d) Manual focus on the Sigma is for ****! Its just AWEFUL.
e) The Peleng takes a bit figuring out if its your first manual focus lens, with a 2-stage aperature control (minimum aperature setting + diaphram control), and a focus ring that is easier to use while looking AT IT instead of looking through the viewfinder. But once you learn to approximate subject distance its sooooo easy to dial in focus. I probably miss focus less than 5% of the time now. It really is a breeze. And the manual diaphram control in Av mode is just sooooooooooooooo wonderful.
f) The Sigma doesn't focus well, at least it didn't for me.
g) The Peleng is built like a Sherman M-1 Tank... it'll never die. The Siggy is VERY plasticky by comparison. The Peleng has a cheapish lens cap that the flocking comes off of for many people and pisses them off, I've been lucky, mine is still on it.
h) Sigma has AF, Peleng does not. The ONLY instance that I would opt for the Sigma would be if you NEED AF for such a lens, in reality the DOF is so large even @ f/4 that its not really necessary - but the one instance which I would want the AF would be if I was trying to do some stuff with action sports. Skateboarding, snowboarding, etc... if you are brave enough and have faith in the subject not running into you and your gear being smashed... the AF would be useful to track the subject as he gets closer and closer to the lens.
If you check my PBase in my signature, you will find a shot taken with the Peleng. But the real story of that shot is that it was a 3-shot AEB bracket with the slowest (+2EV) bracket being 1/50, handheld, @ ISO100. I had tried the same type of shots with the Sigma and the results were not to my satisfaction. The Sigma just isn't as sharp as the Peleng.
I can honestly say after shooting with both that the Peleng is the BETTER lens.
It takes more getting used to, that is a given. But the rewards are worth the effort.
Unless you want to have extreme sports kids flying into you and thats what you want the lens for... don't buy the Sigma, buy the Peleng.