It's understandable that many people who buy a digital SLR would not want imperfect photos. Why spend all that money just to get out-of-focus, soft shots? Well, sometimes, it's just fun to think about your photography in a different way. And that's where the Digital Holga lens comes in.
The base Holga lens is just a simple plastic lens with an effective aperture of f8 and a focal length roughly equivalent to 60mm with a manual zone focus. (It's based on the lens design from the plastic Holga film cameras.) The Kitchen Sink kit I tested--available for Nikon, Canon, Sony, Pentax, and Olympus digital SLRs--included this base lens, three close-up, two macro, fish-eye, 2.5x telephoto, and wide-angle lenses. All of these additional lenses just slide onto the front of the base lens.
Since you're attaching the base lens to a digital SLR or interchangeable lens compact camera (lenses for Olympus PEN, Panasonic Lumix G, Sony NEX, and Samsung NX are available), you're not going to get the same shooting experience or photographic happy accidents that you'll get with a Holga film camera. That mainly means no light leaks and none of the effects of using different films. Vignetting, blur, chromatic aberrations, and lens flare are in full supply, though, and should you have or get one of the film cameras (about $30-$40) you can use all of the add-on lenses with it.
Part of the fun and the challenge of using the Holga lens is its limitations. Basically, you pop the lens on, put your camera in manual, and use shutter speed and ISO to get the correct exposure. With its f8 aperture you need a lot of light or a high ISO to get a good exposure. It wasn't uncommon that I needed to use ISO 800 or above--even with a decent amount of light--so it helps if you've got a camera that produces good high-ISO photos. It also helps if you've got a camera with live view because my viewfinder was frequently too dark to compose photos.
However, even if you're mindful of settings or the lens characteristics like vignetting and lens flare, you still might not get the shot you were after. But, again, that's part of the fun and the challenge of using the Holga lens: you won't always get a perfect photo.
Of course, there are several Photoshop plug-ins, like those from Kubota and Alien Skin, or more consumer-friendly options like Pixlr-o-matic and Lo-Fi, and plenty of iPhone and Android apps to mimic the results from a toy camera. And many newer cameras have filters built-in, too. But there's a pleasure in the randomness of the results with the Holga lens you won't get from software, and a certain satisfaction that you didn't need software to create a particular image.
In the end, if you like some unpredictability now and then when you're shooting, the digital Holga lens ($24.99) or Kitchen Sink kit ($108.99) from Holga Direct are definitely worth picking up. They're also inexpensive enough to get for a gift for your favorite photographer. You probably won't use it all the time, but it's certainly small and light enough to keep in your bag for those times when you feel like something different.