One application which has impressed me with metal implementation even on low end HW (intel HD 620) is iterm2[1] terminal emulator.
IMO, it is a perfect example of how normal screen drawing if made using HW accelerated API could make a huge difference. This is especially true with low end hardware e.g. if Raspberry Pi (Broadcom/Videocore) had implicit support for OpenGL from the start by default like Apple has done with Metal, there could have been a decent desktop experience which is not possible with current SW rendering on low power CPU.
GPU rendering talk is getting lost with discussions about High end graphics cards or GPGPU computation, but I think Metal has brought back the focus to general screen drawing and judging from how developers like George Nachman (iTerm2) was able to take advantage of it; Apple seems to have done good with it.