Fun, although not always practical if you are shooting in a fast paced, distracting environment. For me, anyway. I've yet to bring mine out at a wedding, simply because I don't feel confident enough with it yet. I like to use mine pretty wide open, and so that "sweet spot" of focus can be elusive if you are rushed, or working with moving subjects. Here's a couple that illustrates this--although I do like these photos for what they are, the focus is not spot on where I would have liked it to be:


I'm not good at technical writing (or the technical aspect of anything, really), so I won't contribute much here in that aspect. However, I'll give it a shot. The lens works on a similar principal to the fancy tilt/shift lenses (I think), where you can move the focal plane around until you get the area in focus that you want. This is easier to understand if you look at the lens, than me trying to explain it. It takes a bit of getting used to; moving the lens around with your fingers while trying to manipulate the other camera controls can be tricky. But with practice, it gets easier.
The lens comes with a nifty little set of aperture rings, which attach magnetically to the lens. So you actually chose and place the aperture manually, as opposed to turning a dial or something. Like I said, I like to use it pretty wide open for the greatest effect, but it comes with 7 different rings to choose from, or it can also be used with no ring whatsoever for the shallowest DOF. (I told you I'm not a tech writer).
There are a couple of different versions of the Lensbaby, one a bit fancier than the other. I've got the fancy one, I think, the 3G. See the website for more details and a better explanation than I can provide:
They range in price from around $100 to $300, depending on which version you buy and what kind of camera you have.
I find that I have the most fun with it when I'm shooting just for myself, and have time to play around with it. Kind of like a holga, you're never absolutely sure what you might get--the results can be different every time, and that, to me, is the beauty of the thing.
Here are some of my fave shots done with it last fall:

