I have been using Feedwordpress since a while now for a number of projects. My experience is that it works great: FWP gives you a lot of options to customize your aggregation and does so while giving full control to the user.
This also makes it a bit complex to use for more novice users I guess. That's the only serious downside to this plugin: you will need some time to understand and get into it.
As RSS-feed support goes; there is a lot of non-standard garbage out there and FWP deals with -most- of them in a good way.
I authored a plugin for FeedWordpress ( FeedWordpress Advanced Filters ) and found that the structure of FWP is excellent to work with. With a few filter/action hooks I was able to capture most of the work FWP is doing and make my alterations and features to it.
To conclude: This is great plugin for both simply just grabbing a few feeds and showing them to more difficult and complex situations with lot's of feeds.