I worked my way slavishly through the thing in high school. I could play most of the examples in semiquavers at 180-190 bpm at one point, the semiquaver triplets perhaps at 160 bpm or so. I still open it every now and then, but prioritising it is harder these days, even if my technique could use some maintenance.
I actually bought a lot of the Stetina books recently in a fit of nostalgia. I played myself through most of them during those days. The funny thing is to come back to these things years later and hear how your ear has grown: I hear so much clearer what concepts are being employed now than I did then.