I read the mailing lists for Freenet so I have some familiarity, but I don’t bother running a node because of the bandwidth it uses.
On the technical side, Freenet needs to run 24/7 to be at all useful. It can take a long while to get integrated into the network. It’s completely decentralized, so the quality of your connections starts out random. Assuming your node is good (reliably up with adequate bandwidth and plenty of harddrive storage), your node will eventually find a few other similar-quality nodes to talk to.
Important: you want your node to be in “open” or “strangers” mode (or whatever they’re calling it this week) and not “darknet” mode. In open mode, your node will randomly try to connect to other nodes in Freenet. This is the fastest way to get into the network, and is fairly secure if you don’t mind anyone knowing you’re running Freenet. In darknet mode, your node will only connect to nodes you know the codes for. You’ll need to have a node’s user send you this code. This usually means physically knowing someone else who runs Freenet, because no knowledgeable user is going to transmit their node’s code over the internet.
Freenet is still in beta and has frequent updates that are more than just bug fixes. Everytime there’s a significant change, expect the network to “churn”. You’ll lose some of your good connections and will have to make due with less. It’ll recover eventually.
On the political side, Freenet is about the free exchange of information. (Please do not conflate “file sharing” with “copyright infringement”. The former is legal in general, the latter is not.) Their philosophy is that in most places in the world, there are penalties for exchanging some type of information, so the only way to have truly free communication is via anonymous communication.
Freenet is primarily designed to permit anonymous exchange of information. First, by making it as difficult as possible to detect if someone is running Freenet. That’s the reason for a darknet mode–you’re only making connections to someone you “trust”. I don’t know the technical details, but they’re working on making Freenet network packets look like a video or audio stream. Once that is complete, even your ISP will have trouble knowing that you’re running Freenet.
Second, even if someone knows you are running Freenet (which will be obvious if you’re not in darknet mode), there’s multiple layers of encryption. There’s encryption on the network links and on the files themselves. Because of the nature of Freenet, your node will be storing and transmitting files from others, but you won’t necessarily know what they are. Only if you know the decryption key for a file would you be able to decipher it. They are planning a higher level of encryption–connections between nodes will have “onion”'d encryption, so that your node can’t know where a message is coming from or going to. These are their “defenses” against legal liability from running Freenet; you have some deniability that you could know what’s there.
Third, there are no central servers in Freenet. There are “seed” nodes that have the public info on open Freenet, to help newcomers get integrated. But these are only needed for open mode, and the only play a role in cold-starting a node. Once you’re node is integrated into the network, you’ll never connect to a seed node again. In darknet mode, you’re only connecting to your friends. It doesn’t matter what the rest of the internet is doing.
So, why should you use Freenet? Right now, there’s not a strong reason. It’s still going through major changes. They are making regular improvements. They’re working on getting chat and mail functions. If you like running software that’s bleeding edge and don’t mind the corresponding blood, you may enjoy using/testing it. If you want something that’s working and useful now, I suggest waiting until they hit version 1.0 (maybe in 2015 at the rate they’re going).
If you have more questions, I’ll be happy to try to answer.