The ubiquitous Dave describes describes the right user interface for an aggregator:
I've been using an aggregator for over a year which gives you the choice of both the 3-paned view most aggregators present and a Radio-style single page view containing entries from multiple blogs in chronological order. After using both styles for several months I've ended up using the 3-pane view all the time.
The Radio-style view is essentially very limited in usability compared to the 3-pane view. Its ok for reading once-off news items but lack of context of each item, for example you can't immediately relate an item to other items from the same feed, and the inability to return to archived items makes it not so good for information of longer-term value. To begin with I preferred the Radio-style chronological view but I soon began to tend towards the much greater control the 3-paned view provides.