I have used the mylyn plugin before (it was called mylar then), but I never had a project where it could be really useful. But now I that I have, I'm really enthusiastic.
First let me explain what mylyn is. According to their own website it is a: "Task-Focused UI for Eclipse that reduces information overload and makes multi-tasking easy". It does this by integrating with your task repository (issue tracking systems like jira, trac, bugzilla), so that when you need to switch to or from issues it will maintain the context of the issue you were working on, so that when you switch back you will have all the files you had opened and that we’re important then. This will reduce the overhead associated with switching between tasks.
When you setup Mylyn it will use a query that you already use or one you can newly create within the tool, to create a task list that it will keep synchronized with your task repository.
It will do this with little popups in the right corner of the eclipse interface.
You can open tasks as you would other files that you can edit and save. When you save them the changes will be committed to your task repository.
Mylyn will help you with estimating how much time a task will cost, by logging the time you’ve worked on an issue. It is also possible to get an overview of all your tasks per week, with a junit type progress bar indicating the work done.
It will also help you committing your work by providing standard texts to commit your progress on a task to subversion or cvs. It will keep a delta of your changes per task you are working on.
But the best part is that it keeps the context of the issue you are working on. This context can also be added to the task.
This is very useful for example when you are stuck on a task and you want to let a colleague look at the issue. You can then attach the context to a task, so that your colleague can also benefit from your search work. In jira (the task repository we use) this is done by attaching a zip file.
All in all it is a very useful plugin that can save you quite some time and improve your estimates. If you want to try it as well you can use the update manager find the appropriate version on their site.