I can just answer my question myself (maybe somebody will profit from this)
They work great! I just bought one and tried it. Works splendid!
Ah, an the other problem, with the Compact Flash direct slot on a IBM X21:
Now it works great, AFTER activating the PCMCIA deamon. ;-)
I remember now, some half a year ago, saying: "hmm, who needs that PCMCIA cardmgrd deamon... Do I have a PCMCIA card? Naahhhh......"
The cf slot is recognized as another PCMCIA port.
Great, something more that works....