Take a look at what happens when you put a $25 plastic lens on a $1,200 digital SLR.
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 produces heavy vignetting and even in full sun you'll probably need to keep your sensitivity set above ISO 200. The kit I tested 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.
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. I tested with a Nikon D7000 (it's available for several other SLR and ILC mounts) and it might seem a bit backward to slap a cheap, plastic lens on such an expensive camera. After all, the lens produces soft photos to begin with and then add in more softness from noise and noise reduction and, well, you get pretty soft photos. That's not what most people want from a digital SLR, so these lenses definitely aren't for everyone.
While this photo might look like it was taken at night, the light falling on the pot is direct sunlight. The lenses can be challenging to use and you won't always get usable photos, however, a big part of the fun of using them is experimentation. Simply changing your framing slightly can get you completely different results.