I've been using a Holga camera for about nine years. The reason I like to use them is for their picture quality and because they use medium format film. I also like the fact that you don't really know what your picture is going to look like until you develop the film and print the photo.
There are a few factors that provide this element of surprise when taking pictures with the Holga.
The view finder is okay, so what you see through the finder is a close approximation of what you will get on film.
Sometimes the take up real doesn't keep the film tight so the film is not a flat plane. This can you areas that are distorted and a little out of focus.
Since the lens is plastic you usually don't get a sharp focus, but since the film is medium format making enlargements doesn't degrade the image too much. So you get an image that is surprisingly sharper than you would expect.
Due to the plastic lens, there is image fall off around the outer edges of the image, giving a dreamy kind of image that draws your eye to the center of the photo.
Some Holgas have light leaks, so your perfect picture can be ruined by light leaking in, or the light leaks can make a mediocre image much more interesting.
If you are looking for a low cost camera to use that can give you some very interesting images go out and get yourself a Holga camera.