Yeah it all depends on the feed. Look for feeds that display the entire articles and you'll be golden.
as i mentioned in my other post http://www.blackhatworld.com/blackhat-seo/blogging/29014-wp-rss-plugin.html#post280935, there is a service that transforms partial feeds to full text feeds at hxxp://labs.echoditto.c0m/fulltextrss. it does not work with all sources though. ezinearticles.com is a non-working example. but i have had some great success with other feeds.
Just wanted to document that BlogSense now has this ability through scraping.
http://www.blogsense-wp.com/news/autoblog-content-scraping/