Ryzen is really good if you multi-task. Like streaming from the same machine. x264 will utilize all the available cores. Single-Core performance isn't great, and not a whole lot of applications know how to use more than like... 4 cores. I used to have a i5-4460 which just chugged if I tried to do more than one thing at a time. However the 7700k handles things like that fairly well from what I've heard, and seen. That being said, the AM4 platform is going to be supported for many years, so who knows how the second-generation Ryzen will perform.
So in summary, if you stream, get a Ryzen. If you do productivity things, like CAD, get a Ryzen. If you only game, you're probably better off with the i7-7700k since it has wider support in older applications.