I've had this lens for a few months, and I love it.
Firstly, for Syno, it's not a fisheye. The barrel distortion is a bit high at 8mm (but it beats every other 8mm lens around, oh yeah, there aren't any). By 10mm it's perfectly rectilinear.
It flares a tiny bit, but you really have to point it into the sun and have a dark foreground to show it up.
Group photos are fine, but don't use it for portraits, at least not within 1m (and certainly not from 20cm away). Most people end up with "what's wrong with that, I look horrible, delete it!" (although then they were "wow, that looks really cute" when I did the exact same thing to their dog).
It's variable-aperture, but just set it to f/5.6 and forget about it. There's not much point in going to f/4.5 at 8mm (you can handhold it to 1/10s easily, 1/4s if you're steady), and the DOF is so huge, evan at 16mm f/5.6 that there's not point stopping down to f/8, the centre just gets softer from diffraction (although the edges get slightly better. Maybe f/6.3 is the sweet spot).
You can also just leave it at Hyperfocal (5m or so) and switch to MF, using AF if might focus at infinity then you've wasted half your DOF. Or if using AF, point at the ground a few meters away, never infinity.
It's a cool-looking lens, it's got that reverse-telephoto that contracts front and rear to the centre at 16mm, expands out fully at 8mm, so the front element is always perfectly shaded by the hood (just like the 24-70mm L mk1, unlike almost every other zoom). The front element can get dirty, and it took a while to clean (with wider-angle lenses, especially at narrow apertures, front element dust and grime gets more and more noticeable).
The front cap comes off in 2 stages, you can take the first bit off and mount 72mm filters.
Even at 16mm, you get black corners, so you'll have to crop (so it'll end up at 18mm anyway, use another lens for filters).
And the best thing, is that it can also mount on a FF body, and doesn't vignette at 15-16mm (you get a nice circle at 8mm, don't bother that wide). Still, it's cheaper than a 14mm L or a 16-35mm if/when you go FF, even if it's a bit weird...