I loved the Eclipse IDE when I used it in my internship at CA (last summer). This IDE is probably one of the best IDEs for java, given you include the appropriate plugins for your task. Though most plugins are free, some may not be.
By as much as I knew Eclipse before my internship I was impressed, I have simply compiled some simple java code by then. During the internship I learnt using advanced options of external libraries, plugins etc. With a simple plugin setup(just copying some .zip file in a directory, IIRC) you can make it a J2EE studio, much better than other I have tried till that time. I used tomcat plugin too, which did some external invocation of tomcat server etc.
Now I happen to try this new version of IDE 3.1.1, In some 25-30 minutes I am impressed again. Here is a small description of setting eclipse to use ant build file and creat a project from it.
1) File -> New project
Choose Ant project. It will ask you the file locations and do the rest part almost automatically.
2) Setting Up Java etc.
Somehow eclipse detected existence of java sdk 1.5 from my system automatically. ( I wonder what intelligence must be used if not trying some hardcoded directory names etc). But I needed to compile my project in jdk 1.4.2 which was also installed on my system.
As shown in the figure, eclipse gives a simple option in Menubar->Window->Preferences->Java->Installed JRE, where you can add JRE if not automatically detected. And select the default JRE. The project is automatically build from this default JRE.
3) Setting up Ant.
Just go to Menubar->Window->Preferences->Java->Ant, to check if the classpath is ok, you can modify it to add some new files etc. ( I needed to add tools.jar from jdk1.4.2 instead of 1.5.2 so I removed the 1.5.2 tools.jar and added 1.4.2 tools.jar), all is pretty straightforward.
You might also use the option of external property file for ant.
Eclipse has this wonderful feature of building incrementally autoMAGically, So at all times you can see if some error exists in you code without just needing to build again and again. (Yes you can turn the option off too).
For Building automatically Eclipse was using 'Java Builder' by default. It was annoying me with some Class path error, which indicated that ant was not properly used. I just checked Project->Properties->Builders, there I Clicked new builder which automatically opened option of Ant Builder, rest was all straighforward like setting this new builder as default etc.
5) On a Project->Build clean and rebuild I had my project build properly:)
Conclusion Eclipse IDE rocks. BTW I haven't tried JDev from Oracle which my friend Vishal tells me is another great IDE.