The 28 SK is a true gem and is increasingly sought after because with the latest backs it works great again. It is very rare for historic reasons as SK went out of the digital photo business shortly after launching it. Also, because this lens had issues in 2012 with the then-released 80 MPX CCD backs, this lens was soon ostracised of sorts so basically there's not many of them compared to other lenses. Only of the 120 ASPH there are less, I understand.
The lens is an optical masterpiece and sharp to the edges on my IQ4 150, meaning you can create really dramatic architectural photography with it, one notch above in perspective drama compared to the 32 HR with the added benefit of not needing to correct it in terms of distortion.
In sum: It is truly breathtaking optically. Don't forget, It is the only sub 32mm lens with an IC of 90mm which means it can shift on a FF sensor. The 23 HR has no shift room whatsoever. I don't have a calculator at hand, but I'd assume a 15mm left / right stitch in portrait orientation would be significantly more dramatic than a 23 HR - with great sharpness to boot unshifted and excellent sharpness still when shifted.
It is also rather heavy with many lens elements, so clearly a special flagship / "tour de force" kind of product for SK as well.
There is an ultra rare version - the Alpa Helvetar 28 SK. It came with a special filter ring going to 95mm which allows you to use special effects filters or B&W and infra filters. This special filter ring was made for Alpa by SK, so there's not many of these around. Imagine an achromatic with this lens and a red filter ...
So it goes like this:
1) SK 28 XL rare
2) SK 28 XL Cambo anniversary addition very rare with a silver ring
3) Apo Helvetar 28 XL with custom made SK filter ring very, very rare
2/3 with CF - unobtainium!
I think this is one of these lenses which will be worth a lot more in years to come as Rodenstock planned a sub 32mm >70mm IC lens, but this one won't be produced now due to other priorities. So there's effectively no lens like it in the sense that you can create a shifted look of a building with it that you can't otherwise while still using the full res of the sensor.
I might put up a review at one point, more for historic reasons as it is exceedingly difficult to find one and I searched for a long time.
They cost sub 5k at one point, but are quickly moving up directionally 10k courtesy of rarity and the upcoming Hassy 100 MPX back for which it should be the perfect archi lens jointly with the 43 XL since you do not need to correct for distortion. There was this portrait in medium format mag of that excellent archi shooter and he used the 43 XL and 28 XL on an Arca R camera.
Truly great and there's a reason the 28 XL and 43 XL are dream lenses for architecture with the newest backs - there's just something about not needing to correct for distortion on a non-XT body ... if your business is photographing buildings! Compared to the 35 XL, the 28 and 43 are last gen SK, meaning one notch above even in sharpness with huge ICs, respectively and sharpness to the edge. The 43 XL has an IC of 120! Go figure.
The Rodie 28 has a small IC, I think, so the whole special thing of the SK28, namely that you can shift it quite a bit to create dramatic shots, is not available. I would also assume that although sharp at the center, you need to correct the distortion and that on a full frame you might see quality loss at the edges. I never owned one, so can only assume this based on what I've read about it and based on its specs - it is a Digaron-S, so initially designed for the first-gen of smaller size DMF sensors...