Simon Peter Chappell posted a blog entry on "The Problem with JUnit". The title is a bit misleading, since Simon doesn't really have a problem with JUnit per se. His concern is that this tool/framework is so ubiquitous in the Java world, that people new to unit testing think that by simply using it, they're done, they're "agile", they're practicing TDD.
Simon's point is that JUnit is just a tool, and as such it cannot magically make you write good unit tests. This matches my experience: writing unit tests is hard. It's less important what tool or framework you use; what matters is that you cover as many scenarios as possible in your unit tests. What's more, unit tests are definitely necessary, but also definitely not sufficient for a sound testing strategy. You also need comprehensive automated functional and integration tests, and even (gasp) GUI tests. Just keep in mind Jason Huggins's FDA-approved testing pyramid.
Simon talks about how JUnit beginners are comfortable with "happy path" scenarios, but are often clueless about testing exceptions and other "sad path" conditions. This might partly be due to the different mindset that developers and testers have. When you write tests, you need to put your tester hat on and try breaking your software, as well as making sure it does what it's supposed to do.
In the Python testing world, we are fortunate to have a multitude of unit test tools, from the standard library unittest and doctest to tools and frameworks such as py.test, nose, Testoob, testosterone, and many others (see the Unit Testing Tools section of the PTTT for more details). There is no tool that rules them all, such as JUnit in the Java world, and I think this is a good thing, since it allows people to look at different ways to write their unit tests, each with their own strengths and weaknesses. But tools are not enough, as Simon points out, and what we need are more articles/tutorials/howtos on techniques and strategies for writing good tests, be they unit, functional, etc. I'm personally looking forward to read Roy Osherove's book "The Art of Unit Testing" when it will be ready. You may also be interested in some of my articles on testing and other topics. And the MailOnnaStick tutorial wiki might give you some ideas too.