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Log4j has been around for a long time, and it works very well. I have no scientific study to back it, but based on what I've seen at a large number of clients, it is easily the logging framework that I see used more than any other. It has been around for a long time, and not been replaced by the Next Big Logging Framework, which says something.
It is dead simple to set up, and easy to learn the basic appenders (outputs). There are a whole host appenders that are available, including:
Plus others. It isn't difficult to write your own appender either. Additionally there is a great deal of flexibility in each of the appenders that allow you to control specifically what is output in your log.
One note, I had a series of classloader problems when I used apache commons logging in addition to log4j. It was only for one specific application, but I found it simpler to use log4j alone, rather than to have the flexibility offered when using an abstraction layer like commons logging.
See this article for more details:
Good luck!
I would go with log4j. The possibilites with log4j is not obsolete at all!