The Log4j project has become the de facto logging standard in the java community.  Even with the introduction of built in logging in J2SE 1.4, Log4j still remains the premiere logging framework for java.  The built in library java.util.logging provides convenience but lacks the features of the more mature Log4j.  However, not including the extra appenders and formatting that Log4j offers the function of the two libraries are very similar.  In my opinion you are much better off using the Log4j framework from the start and sticking with it.  The extra functionality has been worth it, at least in my experience.

Another advantage of Log4j is that there are versions of Log4X available for almost every major language.  I also use Log4net for my C# development and it is very nice to be able to switch languages but maintain a very familiar framework.  Almost all the configuration and even the method calls and classes remain virtually unchanged.  One caveat is that Log4net is in some kind of purgatory at the Apache foundation waiting for official project status.  I notice most of their releases are labeled as beta but I have not had any issues with it so far.  Your mileage may vary.

When you need logging, Log4X should be your first choice no matter what language you are using.