I'm one of the committers to Castle project (which includes Windsor) so I may be biased, but.
Windsor is very extensible, allows you to tune and bend it to your needs. It provides nice fluent registration API so that you don't have to write ton of XML. There are also tons of extensions that you can use to integrate Windsor with other frameworks, like ORMs communication frameworks (WCF, NServiceBus) etc.
Plus it has a very active and responsive community, so if you have any questions, they'll get answered fairly quickly most of the time.
