I recently started playing around with writing an Eclipse plug-in, and I thought I should share some first impressions about how easy it was.
The motive was to more easily play with metamath, a system to automatically verify (not write) mathematics proofs. Unless I've missed something, there is little activity on development environments for math proof systems, but it seems to me the need for good tools (like eclipse) is at least as great for math proofs as for software.
In all cases this was based on the Eclipse which ships with Fedora 8, although I'm not aware of anything Fedora-specific. I started with the eclipse help files for "Plug-in development environment". It was relatively easy to create a project seeded with one of the example plug-ins which ships with eclipse (in my case, first the hello world one, and then com.example.witheditor which seemed like the most relevant to the plug-in which I was trying to write, which at least at the start will be a few simple decorations on top of text editing, similar to an emacs major mode or one of the syntax coloring modules for vi, gedit, etc). Generally, the help files walked me through all I needed to get started. I was somewhat puzzled with "how do I get back to the Overview once it is closed" until I figured that opening the plugin.xml file gives you a specialized view, including Overview, with links to click and forms to edit.
The fact that Fedora ships with the source code to the standard Eclipse classes that a plug-in needs to hook into, combined with the good Eclipse features for navigating Java code, made things really easy. I was pleasantly surprised the first time I control-clicked on an Eclipse method and got not only the arguments I needed to pass in, but also the commented source code. Within a few hours, I had turned the example plugin into something which was at least starting to understand metamath syntax. Not bad considering that this includes a fair bit of experimenting (e.g. playing with foreground and background colors) and learning about the platform. I tend to find that Java and Eclipse make it easier to explore a large unfamiliar codebase, compared with a language like Ruby (where exploring large unfamiliar codebases, like Rails, has been a daily activity in my recent job), but I also give credit to developers of the Eclipse plug-in system. The example pointed me to the relevant parts of the plug-in libraries, and the well-commented source code of the libraries themselves helped me poke around to figure out what pieces would do what I want. Another huge win is the way that the eclipse plug-in development environment just worked out the box. There was no messing around with CLASSPATHs, jars, ant and similar rigamarole: just go to the Overview page and click on "launch" and you are running.
Error reporting was a problem: one of my first edits to the example passed a bad value to a constructor in a constant (this is the RGB constructor in IXMLColorConstants if anyone is following along). There was a dialog box referring to an error log, but I have no idea where this error log might live (apparently not in my plug-in project). Ideally, the plug-in development environment would have somehow showed an exception with a stack trace, or something of the sort.
My plug-in so far can be found at mmclipse.