I am running Ubuntu 18.04 on a fairly powerful PC (Ryzen 2700x and Vega64) I finally managed to get OpenCL to work at all with ROCm-rehl drivers.
When I render the default cube scene CPU rendering takes 7 seconds while GPU rendering takes crazy 40 seconds. both are compared using one 512*512 tile.
Also I can't really hear my GPU fans revving up as I can with my CPU fan when rendering on CPU.