We use the Enterprise Library exclusively in our corporate environment (Logging Block). And I've recently started to use Log4Net on personal projects.
From experience, the Enterprise Library, albeit powerful, is a mighty pain to get up and running. Not only from just trying to figure out which dll's to use, but the configuration can become unwieldy.
On the flip side, I've found Log4Net to be extremely easy and you can be up and running in no time; quite literally in less than an hour.
Granted I'm a MS fanboy (ok, not really, but they are the reason I can make a living) and typically go with MS. However, I've made the recommendation to migrate to Log4Net at work. The simplicity of it really is refreshing.