>I've always used makefiles for my Java development
Make is great when you're developing on Unix. But try and move a makefile for a complex build from Unix to Windows and see how well it works. If you're using Ant, and you've done a decent job with your build.xml file and aren't calling too many "execs", the ant build is quite likely to work with minimum effort.
Ant is a cross-platform version of make, and by cross platform I mean the your build process can work the same on many evironments with little effort. Sure you can get gmake for windows. But then you'll need to get bash, cp, cat, rm, etc for Windows too.