I have used the htdemucs_6s a bunch, but I prefer the 4 stem model. The dedicated guitar and piano stems are usually full of really bad artifacts in the 6s model. It's still useful if you want to use it to transcribe the part to sheet music however. Just not useful to me in music production or as a backing track.

My primary use is for creating backing tracks I can play piano / keyboard along with (just for fun in my home). Most of the time I'll just use the 4s model and will keep drums, bass and vocals.