Here is a review on the 35 summarit, 35 V4 cron etc. Its worth a read:
http://www.l-camera-forum.com/leica-forum/leica-m8-forum/38846-summarit-series-35-50-90-ugly.html
It seems plenty have used the Summarit, though not all can of course compare directly to the asph. From what I have read, rather than personal experience, the below would be my rough understanding:
Summarit marginally smaller (diameter) and slightly lighter than asp
Rit takes threaded hood, which attached makes is a hair longer than cron asph with hood.
Rit has lower contrast and colour saturation, but according to some, slightly better flare resistance.
Rit has marginally lower performance at f2.5 compared to cron asph, largely in off axis resolution.
By f5.6 performance is essentially indistinguishable and at F4 there is not really anything in it.
Rit has no focus shift at all, whereas asph has a little at f4-5.6.
Rit has somewhat smoother OOF areas - nice and creamy.
The Rit has more of an old school creamy look but still with moderately high contrast, but not ZM/asph like contrast.
Sounds like an ideal alternative to a V4 cron if you dont need f2.
