Specs are quantitative aspects of the option such as the price
Pros
Pro
Widely Used
Using log4net is a safe bet - many other open-source projects use it as well and it'll be just another dependency you don't have to worry about. This also means finding support to fix bugs is a lot easier!
Because of the poor documentation and difficulty of configuration you might end up spending a lot of time just trying to fix things that aren't working.