My opinion is based on that I have used Both JBuilder (since JBuilder 4.0) and Visual Age for Java 3.0/3.5.
I currently use on a daily basis JBuilder 6. Before I get to that I will give you my input about VAJ. I think VAJ is a great IDE enviroment, very fast, vary intuitive. Helps build your apps quick. However it does have a high system resouce requirement, but that is not what made not use it for production enviroments. I chose not to use it because it forces you to use an IBM programing paradigm, that I do not like. It puts so much propritary VAJ code and tags in your code that your files are larger than they need to be, and very difficult to read.
Oh chorse IBM's VAJ does not expect you to read the code in a text editor. They expect you to use there enviroment, which you never see all this garbage. However if you are going to use some of IBM Enterprise software like websphere, then you best using VAJ. If you are not locked into IBM Enterprise software, then I would recommend looking at other IDE's.
As to JBuilder 6. Because it is written in Java it is a little slow on the start up, and can be slow at times in areas of the program, and because it is a large enterprise level java app, it consumes large abouts of system resources. The faster the machine, and the more memory the better. At my previous company I used JBuilder 5 Enterprise on a DUAL PIII 850Mhz system with 784MB of RAM, and a 7200 RPM 60GB Hard drive running Windows 2K Pro. Under this config I thought it ran like a TOP most of the time.
Now I am running JBuilder 6 on a single PIII 850Mhz, 384MB Ram, and 7200RPM 30 GB HD, using Windows XP Pro. I can tell the performance deference, but I am still very happy with it.
What I like about JBuilder is that it is CROSS PLATFORM, and it does not build propritary code, and it lets you configure how it formats the code, where it puts the currly braces, how it handles listerns, and members. I like the that it published it's open tool api for creating PlugIns. I uses several of differnt ones. This way if JBuilder does have or do what you want, maybe someone was written a plug in to do this. Borland has a large DB of these tools that people publish there and they are free. You can find other that cost money via internet searchs. If you can find one that does want you want, you can just write one yourself.
I think JBuilder has always been one of the top notch IDE's for Java, but with JBuilder 6 Enterprise I think they just qualified as the BEST in my book. The cross platform is nice and I have used it on Linux a few times, and I like that the LnF is the same and I can get right to work, no learning curve.
I have never used Forte yet but I have heard some good things about it. So I can not give any input on this IDE. But these three are the BIG IDE's for Java right now, and they are more of as IDE/RAD/CASE tool which is what seperates them from just plain code editors.
Anyway that is my opinion through into the mix...
Rodney