I really like the Apache Felix tutorials. However, I think in general leveraging OSGi in your application isn't one of those "let's use this framework, because it's hype" decision. It's more of a design question, but then everything that OSGi gives you in terms of design, you can have with vanilla Java as well.
As for the runtime, you cannot just add an existing application and make it OSGi enabled. It needs to be design to be dynamic. Spring DM makes it easy to hide that from you, but it's still there and you need to be aware of it.
