J-Flickr provides shortcode (e.g. [flickr key="value"]) syntax access to the Flickr API.
You can use any method of the Flickr API that doesn't require authentication. Some methods make more sense than others to use, if a method doesn't return any output then you're using the plugin for the wrong thing.
There is a 'templates' directory within the plugin directory that contains a selection of XSL templates. The included ones are for the (what I would imagine) are the most used result sets but making your own is as easy as creating a valid XSL template and dropping it and then using it (see the Examples section).
If you wish for a certain method to always use a certain template just name the template after the method sans the 'flickr.' prefix, e.g. for the method 'interestingness.getList' name your file 'interestingness.getList.xsl'. Case sensitivity will be based on your hosts file system.
If you define a template in the shortcode this will be used before all others, if one is not defined or the file does not exist then the plugin will search for a method template, if that doesn't exist it will fallback to the general templates. The 'photos.xsl' template is the standard and should not be deleted.
The cache exists to be friendly to Flickr, they've been awesome enough to provide a solid API so you should be friendly back to them by not hammering the service. Essentially every shortcode instance you put in your blog is an API call, if the cash didn't exist then every request for a page with the shortcode would be an API request. So say you have 5 front page posts each with one shortcode instance, if 1000 visited in a day, thats 5000 API requests just for the homepage.
You are responsible for managing the number of connections and queries to the Flickr API
I've set the default to be 1 day as that's plenty of time between requests. The higher the value the better really, so if you only update your Flickr account at weekends then feel free to set it to 5 days (432,000 seconds) or longer.