Good or bad is such a limited way of looking at toy cameras with lenses that don't conform to the standards of more expensive solutions. Good or bad is entirely a personal opinion, it would make more sense to describe just what about the camera/lens that isn't desirable. That could actually be helpful to those that are curious.
Both Holga and Diana cameras are wonderful in that they remove a lot of barriers. Whether you know much about using a light meter or not, in 'automatic' mode you get roughly 1/100s at f/11 or so. You can't adjust it, so you don't have to worry about it. One less thing! Focusing is not very critical due to the slightly wide angle design and small aperture. All you really have to worry about is content and composition. It's very refreshing to use, and takes photography back to its simplest elements and you can focus on the single most important thing - subject matter, and making it interesting. That to me is a VERY GOOD aspect of the Holga, or the Diana, or that newfangled 35mm TLR from Japan, etc...
And as far as the results go, I have some negatives that I consider just as successful as my best done with my Hasselblad, and they print just as nicely. All that for $20 or so. What's not to like?
I suppose if you're allergic to light fall-off and completely unsharp corners there's little that can be done about liking it. But in my own, very personal opinion, if you rate how good a camera is simply by resolution / sharpness / perfection - you're missing the point. It ain't about the camera, but rather about your own skill in using your camera. Look at some of the work Jon Shiu has done with toy cameras. To me they're not much different from his other photographs as far as being creations that peak my interest and make my heart pound faster when I see them.
Any camera can be 'good'.