Many webmasters who actually do SEO: study and research it, build links, create original content … are having difficulties with Mashup … they are murmuring in forums about their difficulties.
Mashups work because they are well themed around a topic they contain links to the topic with a small description.
If you look at almost any blog the category page is themed in the same manner. With careful use of the more tag to create continue reading links in the category page this can be used to the creator’s advantage.
Update: Let me add that a custom category page is a must if the category page is to be used this way.