How solid is the build quality?

I'm currently using a 1DX and would like to have a lighter APS body for telephoto lenses. I feel hesitant about the M6 because it is said to have a polycarbonate body. I'm wondering if I could use it confidently with an EF 300mm F/2.8 lens (plus adapter). I understand that I should always support the lens with such a setup. But I think one accustomed to bigger cameras may still accidentally raise the combination by only holding the camera, since the 300mm is very handholdable. Any comments and suggestions from users of other EOSM cameras are also welcomed. Thanks!

Welcome to the DPR forums.

Although it is not the 5 lb weight of your 300mm f/2.8, I am getting very comfortable handholding my 100-400mm L on my M5 (see this thread ). I do have to say that using the EVF and a 3 point stable stance (2 elbows on chest, camera to eye) does help a lot in this regard, something you would not have with the M6. The one ergonomic "complaint" I do have with the small camera and heavy lens is the base of the thumb on my shooting hand, which wraps slightly around the side of the camera, tends to hit the MENU button while shooting. I do have to consciously keep my lens supporting hand in control of the balance on the lens to avoid this. Though I do suppose I will be able to avoid this in the future with practice.

I have no complaints about the build quality of the M5.

The only other thing that people who see you shooting with this combo may notice is that the tiny M camera looks like little more than an oversized rear lens cap on the big lens.

Assuming the M6 will have the same, I'd say the build quality of the M5 is very good, but quite noticeably less heavy duty than the 1dx, which is the gold standard for bq.  It's also noticeably less than the 5D series... The first gen M was so solid and compact that I'd say the build quality was higher than the M5... and the first gen lenses were very impressive in bq, especially when compared to the later M15-45, which seems light and plastic.  I agree w VisionLight that you should consider the built in EVF of the M5 for stability alone, but I have to imagine that w the 300mm you'll be shooting outside, where an evf is a must for broad daylight.

I would treat M series like regular Rebels, just be careful and wipe water, moisture, sand off right away with cloth and blower. Canon DSLR are well built, but never shot in heavy rain / snow with rebels.

It's possible in rain and snow but nothing heavy, a few drops is fine.

Or it's very expensive repair because water damage is not considered manufacturer defect.

There are also inexpensive cases made from Korea, China that may work with M series. The original half case by Canon is okay, not great but improves the grip by a little...

basis unus wrote:

I'm currently using a 1DX and would like to have a lighter APS body for telephoto lenses. I feel hesitant about the M6 because it is said to have a polycarbonate body. I'm wondering if I could use it confidently with an EF 300mm F/2.8 lens (plus adapter). I understand that I should always support the lens with such a setup. But I think one accustomed to bigger cameras may still accidentally raise the combination by only holding the camera, since the 300mm is very handholdable. Any comments and suggestions from users of other EOSM cameras are also welcomed. Thanks!

I've handled the EOS-M5 and it felt very solid.  It certainly felt nice in the hand.  I am purchasing an EOS-M6 as soon as they become available next week.  
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I like Polycarbonate construction.  I hope you're right about the M6 having a polycarbonate body although the top and bottom appear to be anodized metal. Polycarbonate is literally bulletproof (to a 12 gauge shotgun blast at 20 feet)... which is why some types of body armor use it... especially protective glasses.  When dropped, it flexes slightly instead of breaking or denting. And it is less prone to the effects of High and Low temperatures.  There's also less thermal contraction/expansion and screws tend not to unthread with vibration. Another benefit is that the 'plastic' doesn't show paint-loss and scratches like metal does.  The original EOS-M has an alloy frame beneath the outer shell and this is what anchors the Lens Mount.  The new M5/M6 will probably have a similar construction.  The highly regarded EOS 6D Full-Frame DSLR is made of Polycarbonate components over an allow interior frame... and this is very similar.  
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You'll have no trouble using it with any large EF lens, especially with the new DPAF sensor on the M5/M6. I used a number of large and heavy lenses on the original EOSM without issue for some years.  You should NEVER use a large lens on even a high-end DSLR without supporting the glass or the lens mount may be affected.  An exception to this rule for me is when the camera is held around my neck by a strap because I will sometimes allow the lens to hang down towards the ground from the camera in this position.  The rule of supporting larger lenses applies to big bodied full-frame cameras too.  The EOS-M is so light by comparison that it balances nicely on a large lens by shifting the center of gravity.  If used on a large lens that is not mounted on a Tripod or Monopod, you will find yourself holding the lens with your left hand and operating the camera controls with your right.
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The new EF 70-300mm II lens is light and affordable.  Or did you mean the much larger white-bodied EF 300mmL lens?