Just back from a quick series of infinity landscape test shots, somewhat into the sun (lens shaded by hand), f 11 because that's what I shoot 90% of my landscapes and industrials at.
25 mm. edges ok but corners are a bit soft - useable but frustrating for the cost of the lens - exactly as Lloyd Chambers reported. Mind you my Zeiss 15 wasn't perfect, but this is worse - useable, probably ok in most prints, and better for the auto correction in Lightroom, but not ideal.
35 mm. - sharp corner to corner - nothing to criticize.
75 mm. - sharp corner to corner
120 mm. - tack sharp - the best I have, and contrasty, which the other sharp lenses aren't
150 mm. - sharp
200 mm. this is the old version, one lens element less - and sharp only in the centre - not even edges (which is fine, it didn't fit in my bag).
300 mm. - sharp , though a bit low in contrast. Can't comment on corners as lack of depth of field prevented checking, even at 300 yards.
I don't have the 55 or 90 - can't see paying for the 90 when the 120 is so good, and do I really need something between 35 and 75 - so far my experience says no.