I have the 21, 25 an 28. All are excellent in terms of image quality.
The 21 has particularly firm focus, and the aperture ring clicks firmly as well.
The 25 was my general wide angle on the M9 for a few months, as I really liked the extra field of view compared with the 28.
The 28 is my oldest (2008) and most used RF lens. It appears to have developed a little wobble in the aperture ring and the focus ring is not as firm as the others. Sometimes it has a 'catchy spot' in the focus near to infinity, sometimes it doesn't.
I don't know if this increasing looseness will appear in my other Zeiss lenses as I continue to use them, but I hope not. The build quality is otherwise excellent.