JNA seems a fair bit easier to use to call native code compared to JNI. In what cases would you use JNI over JNA?
It's not a direct answer and I have no experience with JNA but, when I look at the Projects Using JNA and see names like SVNKit, IntelliJ IDEA, NetBeans IDE, etc, I'm tend to believe it's a pretty decent library.
Actually, I definitely think I would have used JNA instead of JNI when I had to as it indeed looks simpler than JNI (which has a boring development process). Too bad, JNA wasn't released at this time.