I got one about three months ago and the images are interesting and a little unpredictable. I've tried black and white and transparency with mixed results. Using a tripod you can get some fairly sharp results. At other times I've gotten impressionistic results from accidentally bumping the tripod. I've tried pinhole in 4x5 and 8x10, but the Holga is a good way to go and makes interesting pictures.
I just ordered one yesterday from Freestyle and hope to start playing with it by the end of next week. I have a regular Holga which is a lot of fun. My only pinhole experience is with my 4x10 PhotonBox which I dearly love but I am limited to contact prints since I have no way of enlarging negs that size.