I bought one recently to replace my Voigtlander Ultron 35/1.7, simply because I've always wanted one.
I did a bit of research before buying, and it seems that although the optical formula of the pre-aspherical remained the same, the lens mount changed in 1966 (serial No. 2,166,701 and above), and these later lenses are apparently less flare-prone owing to improved lens coatings.
The first time I used my Summilux, I found it flared much more than my Ultron, even af f5.6-f8 and using the correct round vented hood - admittedly I was 10,000 feet up a mountain with lots of glare! Taking off the UV filter and using my hand as an additional shade alleviated the problem temporarily.
I decided that the although the round hood looks cool, it's rubbish and barely works - and that the Summilux really needs a decent hood to stop flare. I bought a rectangular 12524 rectangular hood, which is nice and small and works much better, and which now lives on the lens.
The rear element is protected by a metal shroud. Unfortunately, it seems that that Leica made a minor change to the shroud length at some stage, so the shroud on certain Summiluxes fouls the bottom ledge on the inside of the M8, preventing the lens being focused at infinity. This was the case with my lens. I have no idea what identifies Summiluxes affected in this way (e.g. the serial number).
The youngest pre-aspherical Summilux is still over a decade old, so I decided that my "new" lens could do with a CLA, - and asked for 1 mm to be machined off the the shroud's height at the same time (cost me £10 extra!). This wasn't done all round the shroud but only for that part of the shroud's circumference encompassed by measuring 7 mm inwards from the bottom of the shroud - this allowed the shroud to clear the ledge without removing more of the shroud than necessary, so the element is still well protected.
12524 hood tips:
• The hood blocks the viewfinder slightly, which might not bother you. It did me, so I made a small cut-out, like Leica does to some of its other rectangular hoods.
• The hood rotates: to stop this, put a thin rubber O-ring (or insulating tape) in the groove on the lens in which the hood tabs locate, to grip the tabs.
• The rubber lens cap 14043 fits, but is prone to falling off. I bent the side tabs inwards, using boiling water, and the cap is now much more secure. If I lose it, I've heard that dense foam cut to size and used as a "plug" is a good alternative.
Sorry folks - typo: the hood is 12524 not 12534. It's for the 35mm/f2.0 Summicron-M.