It's Java Forum next week, here in Göteborg. I'm giving a short talk about TestNG, a tool I've been using lately.
My basic conclusion is that TestNG is a very easy step from JUnit, and one you don't need to take if all your tests are true unit tests (ie fast and independant). TestNG has some nice features which help when your tests are slow and/or have external dependencies, especially if they are mixed together in the same test classes as true unit tests. I think it's pretty useful for unit and integration tests. (aka quadrant 1, technology facing).
Having said that, what bothers me about TestNG is that it means your test code is written in Java. For me, that makes it unsuitable for for system tests, (aka quadrant 2, business facing). If you have anything resembling an involved customer, you're going to at least want to encourage them to read the system tests to verify they are correct, and to gain confidence that the system is working. Truly agile teams have these people helping write tests. Many customer types won't be happy working with Java. You might be able to get by, though, if you have descriptive test names, good javadoc, and test data in separate files that they can read.
Rather than spending time learning TestNG, I think you may get more payback from tools such as Fitnesse, Robot or TextTest, which all allow you to get customers involved in reading and even writing tests. I think it could be a perfectly sensible choice to stick with JUnit for unit tests, and use one of these tools for both integration and system tests. What you choose will of course depends on the situation, for example the size of the system, the nature of the test data, and how many tools your team is willing to learn.