I have battled with getting USB Ethernet to work for a while. I have tried the D-Link DUB E100 adapter, Apple’s 10/100 adapter, and the Plugable 10/100/1000.
I had similar problems and it took me a week to find the solution for my particular problem, thought I wanted to share it.
A while ago I had tried the AX8817x driver from sustworks.com and forgot to uninstall it. It does not reside in the IONetworksFamily.kext/Content/Plugins folder, so I didn’t see it. I got the exact same problems, no DHCP, but link. When using manual IP it showed connection, but no traffic possible. The System Profiler showed the 3.4 driver to be loaded, but I could not see the sustworks driver anywhere.
Once I removed the sustworks driver and rebooted, everything works fine right now.
If this does not fix your problem, maybe it can help some other people.
I also wanted to mention that Bernie seems to give excellent tech support.