I purchased the 2017 27" iMac 3.8 GHz 2TB Fusion Drive model on 8/20 and returned it a week later. In my opinion, the Fusion Drive, even though the 2TB has 128 SSD, was too slow, (read speed around 730 and write speed about 1900 on a new drive) not to mention I didn't like the idea of not being able to take full advantage of the upcoming new OS, as well as having to worry about two different pieces of hardware failing (re: Fusion Drive) and the drive getting significantly slower as more data was added.
On 8/26, I ordered the 27" 3.8 GHz 1TB SSD model. I bought 32 GB of Corsair Vengeance RAM and did the install myself. I have been very happy with the SSD model, even though I initially didn't want to spend the extra money for a true SSD model, when the 2TB Fusion has the 128 SSD portion.
If I hadn't continued reading and interacting on the forum after my first purchase, I would have been unhappy with my initial purchase down the road and would have been angry with myself for not being a little better informed about all the nuances involved.
Edited to add: The Fusion Drive model is noisier than the SSD model during usage (re: video encoding etc.)