Initial Summary
I am primarily a bird photographer, and that is my perspective for these comments.
Build
Flimsier than a 5DIV but still reasonably robust and the ports and doors have seals. Good grip and Canon ergonomics. Easy to hold with a 100-400mm II. Menu is very similar to 5D/7D series.
AF
The centre point is excellent: fast and accurate, with the added bonus of point focus. Just right for pinpointing a bird hidden in foliage or a big slow one in flight.
The centre 9 point zone is not as fast for locking on but still good. I have used it for a dragonfly flying, tracking a heron flying against a background, and it’s very good for finding and tracking birds in flight against the sky.
All 45 points can be very slow for focussing against a background unless the object is clear in the foreground. It will lock on to birds flying against the sky and all the points work.
The focussing is slower with a Sigma 150-600mm C.
Image quality
The quality is excellent. It doesn’t reach its full potential because of its AA-filter. But, it outresolves my 5DSR by about 10-20% with the same lens and clearly outresolves my 5DIV by a factor of 1.5-1.6. In other words, the 90D + telephoto has somewhat better resolution than a 5DIV or EOS R + telephoto + 1.4xTC. It also pairs well with a Sigma 150-600mm C. You get some extra reach adding a 1.4xTC to a 100-400mm II, and definitely gives excellent resolution with a 1.4xTC on a 400mm DO II at f/5.6. In fact, the 90D + 400mm DO II + 1.4xTC seriously outresolves the 5DIV + 400mm DO II + 2xTC.
For normal use, I would restrict the f-number to 5.6 or less and have the 90D as my walk around camera with a 100-400mm II.
For distant birds in good light, I might add a 1.4xTC, which may give some better images but the 400mm DO II + 1.4xTC would be a killer.
Frame rate
I get a burst of 32 RAW shots in about 3s in AI Servo, and then the buffer takes time to clear as I have been using a slow card. I’ll buy a UHS-II.
For me
I am a happy purchaser and recommend the 90D. Until now, my wife and I have travelled for birding trips sharing a 5DIV and 5DSR. But, the 5DSR has generally had the edge on producing good images. From now on, we will be sharing the 5DSR and 90D. The FF of the 5DSR compensates somewhat for the faster frame rate and slightly higher resolution of the 90D. The smaller files of the 90D are appreciated. I would like to see in the future a Canon high density FF with a RAW crop mode.