The 21 S-A is one of the classic Leica M lenses. It is also one where the hype is to some extent justified - it is still a very good lens. So, there is a bit of fall-off at the edges, but that means you never have to edgeburn a print! With a camera like a M2, some Tri X and a couple of red/orange filters - it can do great stuff.
However, if you are looking for images with a modern look, it is not the choice - and truth be told, shooting slides with the S-A can be a bit disconcerting with the dark corners. Here a lens like the C Biogon 21f4.5 is a better choice - and it will work with a metered camera too. Dont discount the compact VC 21f4 either. It falls midway between the S-A and the C Biogon 21f4.5. A bit of edge fall off (though much less than the S-A) but very straight rendition and truly sharp. Some people find it a bit contrasty with color, but in bl/w I haven't had a problem with that. It is also 1/2 price from the 21/4.5 and tiny. It can rattle around in your pocket until you need it.