My Holga is remarkably leak-free for the most part, though I do notice a touch of vignetting in the corners. I don't mind the plastic lens, though I wouldn't mind a proper aperture setting and perhaps a variable shutter. My camera is slightly modified - as stated in my original post, I added a black flap over the film counter window, and I keep the back secure with a heavy elastic. One of the first things I did with my Holga is screw a 46-55 filter adapter into the plastic of the lens housing and added a rubber lens hood. I'm not a fan of scratches, so I buffed the film mask rails to eliminate a couple of annoying burrs which would occasionally mar the film.
I have considered a couple of other modifications, but don't want to mess with things too much. There is the "close-focus" mod in which you replace the plastic focus stop with one a few degrees farther around the barrel, and the "true aperture" mod, but I really don't want to get too far into the innards of the camera... it works fine for me the way it is.