Worked out of the box with SuSE8.2 and kernel 2.4.20-4GB.

The reader is recognized as USB mass storage and will be made availabe as a scsi device (/dev/sda if you have no others). Right after plugging the reader in, I get some error messages in the system log (I think they are related to the fact that the reader is only the interface to the actual "harddrive", the memory card - maybe this will disappear with a later kernel?). As far as I can tell, you can safely ignore these messages (please correct me if you know otherwise!) - just plug in a card, then mount it: an entry for /dev/sda1 might already have been entered into your /etc/fstab by the usb hotplug system, otherwise just add something like:
/dev/sda1 /media/sda1 auto noauto,user,exec 0 0

Don't forget to umount before you take the card out or you might lose data.

Note: I only tested SD cards, since I don't use MMC - but I imagine there shouldn't be any difference. I'll give it a 9 out of 10, because of the error messages, see here for an example:

excerpt from /var/log/messages after plugging in the reader (with a Verbatim 256MB SD card)

Nov 18 12:26:44 kernel: hub.c: new USB device 00:07.2-2, assigned address 106
Nov 18 12:26:44 kernel: WARNING: USB Mass Storage data integrity not assured
Nov 18 12:26:44 kernel: USB Mass Storage device found at 106
Nov 18 12:26:51 kernel: Device not ready. Make sure there is a disc in the drive.
Nov 18 12:26:53 kernel: SCSI device sda: 494080 512-byte hdwr sectors (253 MB)
Nov 18 12:26:53 kernel: usb-uhci.c: interrupt, status 3, frame# 190
Nov 18 12:26:53 kernel: sda: Write Protect is off
Nov 18 12:26:53 kernel: sda: I/O error: dev 08:00, sector 0
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 0
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 494072
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 494072
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 0
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 0
Nov 18 12:26:53 kernel: unable to read partition table
Nov 18 12:26:53 kernel: I/O error: dev 08:00, sector 0
Nov 18 12:26:54 kernel: SCSI device sda: 494080 512-byte hdwr sectors (253 MB)
Nov 18 12:26:54 kernel: sda: Write Protect is off
Nov 18 12:26:54 kernel: sda: sda1
Nov 18 12:26:55 /etc/hotplug/usb.agent[11529]: Can't ignore signal CHLD, forcing to default.

I plugged in my reader properly for sure, But it keeps saying that I have an I/O device Error. It isn't working on my computer. Can anyone tell me what I can do to fix it?

Thanks.