So I'm circling back to this older thread since I just received a rental Sigma 16mm F1.4 today to try on my M6 (along with an RF 35mm F1.8 IS for an RP).
First impressions, it's definitely a quality lens. To cut right to one of the ongoing discussions about lens barrel diameter, I see no particular disadvantage to the wider diameter of this lens compared to the smaller (and identical) diameter of all the current EF-M lenses. If Canon produced wider diameter lenses for EF-M, I'd be fine with that.
That said, while this lens balances "fine" on my M6, it is definitely longer and heavier than I expected. Ergonomically, it doesn't feel quite as part of the camera as the EF-M lenses that I own do (22mm, 32mm, 15-45, and 55-200). Though it obviously wasn't designed specifically for Canon, but rather for Sony and Micro Four Thirds.
I did some indoor and outdoor side-by-side image quality tests with the 15-45mm set at 16mm, and both lenses set at F5.6 (I just picked a generally middle-ground aperture). Maybe the Sigma was a little sharper than the 15-45, but I'd have to really pixel peep at full-size to see it. That was just a preliminary couple of test scenarios, though, around the house, some flowers outdoors, etc. We'll see how it goes for the rest of my time with it.
If you need fast shutter speeds for moving subjects, then I can certainly see the appeal of this lens over the slower 15-45. For non-moving subjects, the image stabilization in the 15-45mm may more than make up the difference for the narrower aperture.
All four of my other EF-M lenses, including the 55-200mm, I would consider "walk around" lenses, meaning I'm happy to hike all day with them. I'm not sure I'd be happy walking around all day with this lens, given its weight and extra bulk. I'll give it a try and see how it feels. For me, being able to walk around all day with an M-series camera is a large part of the appeal. For those that don't prioritize that, the advantage of the faster shutter speeds may make this lens a worthwhile investment. I have it for ten days so we'll see how it goes.
Incidentally, I tried the RF 35mm F1.8 IS on my RP and that lens is *spectacular*. Wow. I've only had that rental one day but I'm definitely buying a new one. The F1.8 aperture combined with the image stabilization is incredible, and the lens is ridiculously sharp.
Photos of the Sigma on my M6:
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Compared to the 55-200 (slight distortion in the photo from my cell phone camera):
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