It depends what the feed is. It's often against their T&Cs to use their feeds in that way.
Assuming you can find feeds that you're allowed to use, you'd need to make sure the content was relevant and useful. Even then, you'd have to consider how much traffic you'd be losing to the feed's website.
I've done that before and its a good idea for a news section maybe. If your forum is about android lets say.. you could use the rss feed to pipe google news about android into your news section. For regular content though, to jump start a forum i've always used sites like postloop to get some good searchable content going...
IMHO the RSS Feeds system only creates clutter, which in turn will drive members away.
It's best to find interesting articles and post them manually, or post the part that is interesting with a link to the source. This filtering of relevant news is what makes forums a very powerful source of quality news.