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Here are three of my SMC-M's.
I love these three and in particular the 35mm. The 35mm was not a big seller and a such is not too easy to find these days but not what I would call rare. Well worth hunting out if you need this focal length. Of course a 35mm lens on a 1.5x crop sensor gives you 52.5mm which is ideal for use a normal lens.
The 40mm is the pancake lens that many lust after although it's not particularly rare but it does command a price just for being what it is. It's not particularly fast either and is prone to flare. The small focusing and aperture rings make it a little fiddly to use as well. I like it though and it goes well my K1000.
The 28mm lens is more maligned. Although not really with any reason. Sure it's a little slow but overall it's a useful lens and is about as wide as you can go without splashing out lots of money for 24mm and wider.
I do have some others to go with them too. My full lineup of M's to date are ........
M28/3.5
M35/2.8
M40/2.8
M50/1.4
M50/2.0
M85/2.0
M100/2.8
M135/3.5
M200/4.0
M35-70
M80-200
My particular favourites are the 35/2.8, 50/1.4, 85/2.0, 135/3.5 and the 200/4.0
But overall the complete line of M's are pretty good lenses and always impress me with their performance.