The process starts simply: you need to make an account at Radar.net, and of course install the app on your iPhone and sign in to it there. At its core, Radar is simply a picture sharing program -- you take a picture on the iPhone, and then can upload it to the site with a short title, where others can see it and even comment on it. In that sense, it's not much different than Twitpic or Flickr or Snap My Life or any of the other photo sharing apps out there.
But where Radar starts to set itself apart is how it connects to all of the other services you're using. If you put in your Twitter credentials, it'll tweet as you upload a pic. If you sign up for their app on Facebook, it'll send your pictures there automatically. And the app doesn't just send pictures out, it can bring them in as well -- you can browse pictures from your Flickr friends by signing in to that service, and even browse a "Gallery" of Radar accounts, some of them from official sites or bands (you can see some pics from one of my favorite bands, Bloc Party, in the gallery above).
In short, Radar wants to be your one-stop shop for picture sharing: take a picture and share it with Radar, and they'll make sure it goes everywhere else you want it to go. But is it worth it? In return, they get to host your content -- every time someone clicks on a link to your picture, they'll see Radar's site and likely get sold something there (even if there are no ads on picture pages right now, as appears to be the case, there's at least a pitch to join Radar). And Radar has even put Admob ads on lots of the app's UI screens: even while browsing my own "posts," I was dodging ads that floated above the UI as I scrolled down the screen.
The app itself is free, and of course they've got to make money somehow. But then again, if I really wanted to make sure my Facebook and Twitter friends knew I'd shared a picture, there are already ways to hook up those two services and make sure my pictures get broadcasted to the right places.
So it's up to you -- as a one-stop shop for picture sharing, Radar can be helpful in tying all of your different services together. If you're sharing lots of pictures from your iPhone and need some help in organizing and sending them out, it could be very helpful. And we also have to mention that the app itself is very solid -- even while flicking through pictures, there was never any lag or big loading waits, and the "fullscreen" viewer is so good and responsive to the multitouch screen that it even beats Apple's official Camera viewer -- you can spin and rotate and zoom in and out with ease.
But the Radar service replicates a lot of what's already available, and though it won't cost you anything, you can expect to spend a lot of attention on the various ads you'll see in the app and service. It's worth checking out if you do a significant amount of photo sharing (and if you have a lot of Flickr -- or even Radar, though the network is fairly new -- friends whose pictures you browse often). But for people who only occasionally twitter about a pic they've taken and already have Facebook hooked up to a Twitter or Flickr feed, it'll probably ask too much for too little.