The only real problem I have with CrashPlan is with massive, regularly changing files. Files like 40-80GB virtual machine images.

I was struggling for a long time to get these to backup properly. Despite grinding away for months on end it sometimes just never managed to get these files backed up.

I think I've resolved this now. For some of the VMs I've switched over to the image format where the drive image is split into lots of smaller files. In other cases I've snapshotted the VM, so there is a large unchanging file and a smaller changing one.

It seems it is the combination of a massive file, that is also regularly changing that gives it problems. I assume these VM images get lots of appends at the end, I think every time it started backing up one of these files it spent all its time wading through the start of the file checking to see if it was backed up and never managed to reach the end before switching away to run another backup set.