I have used three cards so far, a Kingston 2GB card, and two Cisco Systems branded 64MB cards, of which one is manufactured by SanDisk. Both cards were initially written on the PC by means of Drive Image, with the sample 128MB image from the site.
Reading them back on the PC works with no problems. Reading on the Spectrum works flawlessly with the Kingston one, but fails randomly with both of the Cisco cards.
Writing to the cards doesn't work on the Kingston card. Nothing new appears in the catalogue.
At first I thought that writing fails on Cisco cards, corrupting a byte or two in each KB, but after transferring the saved data to PC it loaded back in emulators correct.
After some fiddling with the interface I've built, it became obvious that by touching the PCB tracks "improves" reading to a point that no corruption appears. If I hold my finger across the CF connector, sometimes it would load the file correctly. It must be some stray capacity messing the signals up, because I've designed the PCB in a hurry, and there is a lot of "vias" made from thin enameled copper wire.
Here are the pictures.
Those two resistors are pullups, for which I thought would increase stability, but they didn't work. Also, a blob of solder was used on the CF connector to connect a line that I forgot to connect to ground.