I have a ZM Biogon 25 mm 2,8 which i bought together with my Zeiss Ikon right after the camera and the lenses were launched. It has worked flawlessly. This Biogon design is some of the best wide angle lenses ever made. It draws streight lines and produces both high resolution and high contrast. The lens shade and lens cap is better than on my Leica Summicron 35 mm 2,0. The drawback with this ZM 25 mm 2,8 is that it does not work well with the M8. Even if coded (mine is not) it 'confuses' the M8 with it's cam profile. Has this been changed at later ZM 25 mm 2,8 models..?
Then I bought a 2.hand ZM Planar 50 mm 2,0. After having it for half a year, - it was then two years old, I found that the inner barrel had a slack of half a millimetre or so. It is obvious that such 'slack' is no good for the optical quality. Although, I have not done any tests that really confirms this. I guess that such a slack will first be visible at close range when the barrel is in a far-out position. Also here; the sun shade and lens cap design works equal to or better than with my comparable Leica lenses (well, I have only one Leica 50 mm; the Noctilux).
I had a Canon 24-70 mm 2,8L zoom with the same sort of slack after I lost my 1Ds II out of my car (The shutter went off and that crashing sound of the mirror 'eating' the viewfinder glass was heartbreaking, - but it went well). The Canon service senter here in Oslo changed a plastic ring insde the lens and now it works OK. Still, I am not that impressed by the quality of many Canon lenses, - but they are cheap!