I have had the excellent little M-Rokkor 90mm f/4 (same as the Elmar-C 90mm f/4) for a very long time and have always enjoyed its performance, but I often felt it was just a hair longer than I wanted and a 75mm would do better. So I bought the latest Summarit-M 75mm f/2.4 in mid-2016.
I chose the f/2.4 over the f/2.5 because the close focusing distance on the f/2.4 is shorter, mostly. I chose the Summarit over the Summicron which I tested because I didn't feel I used this focal length enough to warrant almost double the price, and the f/2.4 lens felt nicer in my hands due to its smaller size and lighter weight; I had no reservations about its imaging performance although does image differently compared to the Summicron. I bought Leica vs any other brand because my M camera is now the M-D and I want the camera to automatically recognize and embed the right EXIF data and apply the correct lens profile, and the M-D has no manual profile assignment capability; AND because I just prefer the look, feel, and build quality of Leica lenses over any others.
$1900 was a bit to spend compared to the other brand alternatives, but in retrospect it's been a bargain for me. I've worked with the Voigtländer 75/2.5 and 75/1.8 in the past, on loan, and they simply do not perform as well as the Summarit-M 75. The Summarit-M 75mm f/4 performs beautifully, the supplied metal lens hood unscrews and reverses to make it more compact in the bag if you need that, it includes caps for both use with and without the hood too. It's a premium lens, although in Leica lens terms it's their cost-conscious price leader. It works beautifully on the Leica M-P typ 240 as well:
Leica M-P + Summarit-M 75mm f/2.4
ISO 200 @ f/2.8 @ 1/125
I've gotten so used to the 75's FoV that I fit it to my M4-2 (35/50/90/135 framelines) and just go shooting with it, works perfectly. It's just a matter of visualizing the framelines inside the 50mm. I have also used it fitted with M Adapter L to my Leica SL and it performs brilliantly on that camera too, with automatic identification via the lens code just like the M-D and M-P.
On the wide end of the spectrum, I had the Color Skopar 21/4, the Heliar 15/4.5 v1, and the Elmar-M 24/3.8 ASPH. The Color Skopar was one of my favorite lenses on film and on the Ricoh GXR-M, but doesn't work well on the M's FF sensors (color shifting, etc). The Heliar 15 v1 ... same. The Elmar-M 24 works brilliantly, but I felt a bit constrained by the 24mm focal length and traded it for a second-hand Tri-Elmar-M 16-18-21mm f/4 ASPH (the WATE). This is an outstanding performer at all three focal lengths. It's a true zoom so you can use it as such on the SL, on the M you generally want to use it at the discrete focal length settings. Of course, you need a finder for it ... the big Leica multifinder is expensive and bulky, so I use Voigtländer 21mm and 16mm finders for, just estimating the 18mm setting (all the finders are pretty approximate so I don't sweat it). On the SL, of course, you don't need any accessory finder at all and you get absolutely accurate framing and superb imaging performance at all settings. This is the only lens in this focal length class I'll ever need, so that ameliorates it's rather lofty price tag. Unlike many others at the short end of this range, you can fit standard screw-in filters too using the optional Leica filter mount accessory ring (it comes with hood and hood cap, the filter mount replaced the hood and includes a lens cap).
I've heard varied reports about the Zeiss 25/2.8 on the digital Leicas which were mostly good, but the focal length and negative reports turned me off from trying one. I don't really need this focal length anyway, and found the 24 a bit confining.
My most used lenses with the M are the Summicron-M 50mm, Summilux 35mm v2 (a 1972 version that I had coded), the Summarit-M 75mm, and the WATE. Most of my shooting is with the 50mm or the 35mm, by far; the 75 and WATE spend about the same amount of "other time" on the camera, and my 90, 135, and 28 I use only very occasionally. For travel, the WATE with either the 35/75 pairing or 50/75 pairing is just about a perfect complete kit for me, or just one of the two pairings without the WATE alone if I don't feel I'll use the ultrawide FoV.
Hope that helps a bit.
G
