As both a Balsamiq and JIRA user I'm interested in this plugin.
Has anyone used it? Any idea if it allows you to save the .bmml files in a specific folder, hence allowing the use of project assets?
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Haven't used it yet, so this is somewhat speculative, but as a user of both Jira and Balsamiq, my take on this from the description is:
The plugin allows you to create mockups from within Jira, and probably shows the picture (if referenced correctly) of the mockup in the description of an issue. The latter would be a big advantage (a picture paints a thousand words) as it means you don't have to start balsamiq, nor distribute it to all your Jira users to view the mockup with the issue description.
Also, you probably don't need need to use the plugin to create the mockups. You could simply use the balsamiq desktop app, attach the bmml to an issue, reference it in the text and have Jira show it "inline" in the description of the issue.
I and my product management/user experience team use Balsamiq Mockups on a regular basis; it's a simple and powerful tool. You can indeed save bmml files into whatever folder you like on your hard drive—at least, you can if you have the desktop application; I've never used the Web application.
If you want to get an idea for what Balsamiq can do, you can use the full product for a week before you need to buy.