One thing I heard abt JBuilder is that it puts (read forces) its own classes in the imports. I hate the notion that something I develop is forced to depend on the tool I use. My $0.02 - satya ps: Fortunately, I don't use JBuilder.
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JBuilder is one of the easiest IDE's to use, and I find it's pretty configurable as well.
I've never had it use it's "own" classes.. unless you're speaking of GUI building, in which case, yes, the wizards will use some Borland classes, but no one is forcing you to use wizards. All IDE's are guilty of this, to some extent, particularly for GUI building.
That said, we've built AWT and Swing applications using only classes found in jdk1.1.8 and jdk1.3 respectively.
JBuilder is also a 'clean' IDE because it doesn't stomp on any system classpaths. (take notes webgain)
I have heard that the VisualAge IDE is very complex and unless you are running a P3 with about 1/2 a GB of RAM, it is very, very slow.
For 85% of developers it is probably much more than is needed. On the up side, I have also heard positive reports about its ability to debug remotely and how it debugs threads, which are always very challenging issues for any IDE.