I've been though all the 90's... I really like the 90 AA Cron as well as the 90 Pre Cron, both are supremely sharp at f4 and smaller, and the AA is exceptionally sharp from wide open. However, both are also somewhat large and heavy by Leica M standards and I found that too often I'd leave it out of my bag unless I thought "I'd really need it" ... As a result, it usually wasn't in my bag when I wanted it. Back in the day, I went with a 90 Tele-Elmarit thin for a smaller lens, but this flared like a mother when pointed toward any bright light source. As for the regular Elmarit, it is very sharp, but didn't seem enough smaller than the Cron for giving up the full stop. That is the main reason I went with the Summarit this time around.
The Summarit is sharp stopped down to f4, yet has some of the pre-Cron smoothness wide open. And it only gives up half a stop to the Cron. It's kind of half-way between the Pre and AA in rendering at f2.5 though about 3.5, then from there up, it's virtually identical to them. And the Summarit is still a tad smaller and lighter than the Elmarit, yet half a stop faster -- and there are times I really appreciate the speed in a 90 and figured I'd press f2.5 into service enough to justify the slight additional cost over a used Elmar.
In the end, I am very happy with my decision. Wide open it is sharp while retaining a classic smoothness that works wonders in portraiture, and is then cruciallly sharp when stopped down; best of both worlds. And I use it wide open enough to appreciate the added half-stop
Most importantly, this 90 is ALWAYS in the bag and gets pressed into use quite often.
I do have one cautionary nit: sometimes the Summarit shows a very slight amount of CA along very high-contrast edges. It has yet to be visibly problematic in any web or print image for me, but the fact remains it is visible in some images at 100% view, so be advised...
Cheers,