Basically it allows for selective focus that can be used to direct the viewer's attention to a small part of the image while de-emphasizing other parts. It can be a little hit and miss, occasionally producing some nice 'accidents'. The accidents are at the moment very much down to me and I'm sure with more usage and perhaps changing the aperture ring from the maximum F2 to say F4 will help. Yes, you have to change aperture by physically changing the rings in the lens.
I've been playing with it for a couple of days now mainly to see if I could use it for my professional work, portraits, weddings and engagement shoots. With a little more practice I don't see why not. The images have a rather lo-fi look to them that I really like. As long as it's used sparingly I think it could become an interesting addition to any shoot.