If your project is 'simple', then maven lets you get up and running quite quickly. By simple I mean you have a bunch of code, some resources, some test classes and it all goes together with some 3rd party jars to make an application.
The moment you want to do something unusual that is in some way specific to your own project then you will end up spending all your time trying to make maven do what you want it to and none of your time working on your code. This for me defeats the purpose of using a clever build system.
Maven is also dreadful for helping you diagnose problems when it's not working. And the build scripts are unreadable screeds of unintuitive and unnatural xml, which may of course be what you prefer and are looking for (If you have ant-vision).
I love maven. Maven is full of goodness and promise. I also hate maven.
Edit:
Oh, and the maven plugin for eclipse is flippin' brilliant.
