So I just got one on Friday and did a little side by side test with a VX1000.
GH5 had internal audio on manual at -12db, VX1000 audio on auto. VX in left hand, GH5 in right. Audio is straight from the camera, it hasn't been edited at all.
In my opinion it's definitely the best mic option for skate audio available. It's obviously not 100% identical, but the pop/land/grind sounds seem veeeery close.
I might try to do this again at some point with a Rode VideoMic as well, but its annoying enough filming with two cameras at once let alone adding in a third. If anyone has any other suggestions for tests let me know and I'll see what I can do.