I played with Eclipse 3.0's new RCP (Rich Client Platform) support which allows you to build client software using the same engine that drives Eclipse 3.0 IDE. While one could have build a client using just SWT, RCP allows you to reuse many of the Eclipse plugins in your application.
In essence, RCP allows you to easily repackage Eclipse 3.0 IDE such that it only has the features you want presented in the way you want. Dependencies between plugins will complicate and bloat your RCP application, but I am expecting many RCP-ready plugins to be available soon so this is not a big problem.
The big problem, for now, is the lack of RCP tools and documentations. Eclipse 3.0 has very little built-in tools to help you create RCP applications. For example, Export Plugins feature to export a test RCP app but it didn't automatically include the necessary plugins. So I had to write an Ant script to do that. But then my sighs of annoyance turned to urghs of disgust when Eclipse deleted the Ant file when I ran the Export Plugins again.
I had a lot of trouble running and debugging the RCP app too. I followed the instructions in the RCP tutorial exactly, but the startup code kept complaining that it couldn't find the application. Looks like I'll have to dig into the plugin startup code and understand it more throughly.
In conclusion, I think RCP is wonderful but it's still a bit early. Wait a few more months before diving in unless you can't afford to wait.