I've been using the 6x12 pinhole Holga for a few years now. The 6x12 format is not ideal for a pinhole camera which calls for a more rectangular format. Fortunately the camera includes a 6x9 mask. The default pinhole is too large for 6x9 (appr. 0.3mm) and I changed to a smaller one (appr. 0.22mm) that gives me much nicer results. Other problems I've had are sloppy film winding mechanism which gives you soft wound rolls that leak light and an inadequate, cheap plastic socket for the release cable.
Mablo, where did you get the replacement pinhole? I have been thinking about getting a better one.