Here is a look at the lens and its Waterhouse stops:

The lens is very finely machined and finished. Fit-to-camera (Nikon and Cannon versions are available) is crisp and snug. The rack-and-pinion focus knob—something I expected to dislike—turns out to work easily and well.
The Waterhouse stops—another element I expected to find bothersome—also turn out to be easy to work with. And as a huge plus of Waterhouse stops, no matter what aperture you shoot, the out-of-focus highlights are perfectly round or elliptical—never a visually-irritating polygon. For those unfamiliar with Waterhouse stops, they are a precursor of twist-to-adjust irises; the lens has a slot, into which you drop a flat piece of metal with a hole in it. You carry a separate stop for each aperture you want to shoot at.
But a pocket-full of loose Waterhouse stops is a bother. A good way to manage the stops is to put them on a Nite Ize S-Biner Size 3 (pictured above). With this carabiner hooked to a lanyard around the neck, using this lens is a lot more fun.
Joseph Petzval invented this lens design in 1840. It was revolutionary at the time, offering a maximum aperture as wide as f/3.7—much greater than other lenses then available. This permitted faster shutter speeds, greatly increasing the range of subjects photographers could shoot. During a big chunk of the 1800’s, most photographs were made with Petzval lenses.
Eventually, lens designs moved on, bringing improvements in resolution, spherical aberration, coma, astigmatism and other things. Petval designs were left behind. Yet some photographs from the 1800s have a look that to my eye is wonderful, and impossible to create with modern lenses. Not even aggressive Photoshop manipulation can easily create it. "Swirly bokeh" is a big part of the look.
For a long time, I'd thought about buying an antique Petzval and modifying it for a modern camera. This is certainly possible, but has its difficulties: Lots of antique lenses have serious condition issues, such as lens separation or fungus, and optically good specimens tend to be expensive. Also, most antique Petzvals have longer focal lengths than I'd generally want to use on a DSLR, since they were intended for use with much larger film formats.
So upon learning that Lomography is now selling a Petzval lens of current manufacture, with a lens mount and focal length well-suited to modern digital formats, I hit the “buy” button pretty quickly.
The Petzval turns out to be a lot of fun to use. The swirly bokeh gets addictive, and is surprisingly controllable. If you want a lot of it, set up a shot where your subject has lots of out-of-focus elements at varying distances from the camera--and the busier the background, the better. (How often do you find yourself wishing for this?

For me, this is a lens to use without a tripod. Ultimate sharpness isn't the point--you're playing with the blurry bits. When the lens is focused close, these change a great deal with different framings, so it's nice to be able to experiment quickly. With the Petzval, I find myself dancing with the negative space in my photographs. Though I'm not much of a dancer, I'm enjoying it just the same.
For close-up images, the lens works just fine on an extension tube.
I can highly recommend Lomography Petzval lens for photographers whose interests are well matched to it. This said, another lens deserves mention in the same breath: The Zenit Helios 40 2, 85mm f/1.5. This is of design later than the Petzval, but still not modern. It can give swirly bokeh (though less of it than the Petzval), but with far greater sharpness away from center than the Petzval. I bought one of these because I like the Petzval so much; the Helios would seem to be similar medicine, but at a lower dose. So I think they'll both have a place in my Pelican case. (I should mention that I haven't spent enough time with the Helios to really judge it, though my initial sense is very positive.)
More views of the lens here.
--Chris
--edited typos