The newer XU1541 device connects a IEC device like a 1541 directly to USB. With the current CVS version of
OpenCBM you can use this device to copy files, format disks, write images on the command line.
Here are the details:
http://www.trikaliotis.net/xu1541You have to built this device yourself and compile a CVS version of OpenCBM for it.
I did that and can say that it really works fine on a Mac.
With a RRNet+MMCReplay cartridge you can directly connect your C64 via ethernet cross-cable to your Mac. With
DiskImagery64 on your Mac and the
Final Replay ROM on your MMCReplay you can easily transfer disk images in both directions.