Here's my initial take on a rental 5ds (no "r"):

I've had it for 3 days of mostly miserable sunny weather, so I've only had about a half day of suitably cloudy skies to work with. Still, I've managed to shoot enough frames to get the microfocus set with both extenders and, believe me, this is something you have to do, if you are going to get any worthwhile images from this camera - if you don't, you might as well save the money and get a cheaper model of 20Mpix or less. I've done it 3 times now and still think that it's not quite right with my 2x (the sun and heat don't make it easy to adjust focus at 50x the 1200mm focal length!)

Dynamic range seems to be significantly less than my 7dii and noise seems to be more intrusive at ISO 1600 which is needed a lot of the time to get at least 1/250 sec. at f/11.

What I was hoping for with this camera was at least comparable image quality with the EF 600/4 for birds (allowing for the higher pixel count to off-set the loss of magnification) and still be able to do great landscapes. I don't think that's happening due to the dynamic range and noise - I think the 7dii is still better for birds and I don't need a landscape camera at the moment.

By the time my week rental is over, I hope to be able to shoot it and the 7dii side-by-side on a cooperative bird for direct comparison. Here's hoping for more cloudy skies and normal temperatures.

The last day of the rental is over, it's in the box.

Today I shot RAW and maximum size JPEGs simultaneously and the JPEGs did not fare well for me. They are slightly soft and low contrast, meaning that they would need additional sharpening and contrast adjustments and be just that much less useable. The only gain would be in-camera and editing speed compared to shooting RAW, but there's no speed gain compared to the 7dii.

I'm not planning to shoot landscapes or still-lifes again and, if I was, I'd probably get out my Pentax 67 and test all those rolls of 220 Kodak Portra that are in my freezer. I'm just not inspired to do so. Birds are a lot more entertaining and don't require more than the minimum travel around my part of the county.

Like my previous trial of the Sony A7r last year, I keep hoping for an ideal system with better resolution than I've got, only to be stymied because of the limits of my work habits, subjects and lenses.

I've just posted a "mini review" of the 5Ds (no "r"). I was impressed with it as a camera for landscape, portrait and commercial work where you can take time to optimize focus and you don't need the fastest possible frame rate. See http://www.bobatkins.com/photography/digital/canon_eos_5Ds_review.html

However for nature and wildlife, I agree that the EOS 7D MkII is probably the best bang for the buck right now if you don't absolutely NEED full frame. As Frank points out, things like atmospheric turbulance and slight focus offsets can negate the higher resolution of the 5Ds over the 5D MkIII, assuming you can get the framing you want without cropping. If you have to crop, then the 5Ds is better then the 5D MkII, but probably not as good as the EOS 7D MkII.