I am currently using log4net on a reasonably sized Windows Forms project. I had previously used (in my Java days) the precursor, log4j, so I didn't look beyond log4net when considering a logging system.
I found it very powerful to be able to put all the debugging in place permanently and then enable/disable as required when debugging specific sections. Also if a problem is found on a customer machine that we cannot replicate, we can increase the logging level on their machine in an attempt to debug the problem. This can prove invaluable.