I have both. The UC-Hexanon, being a wide angle and f/2 Xenotar in design, exhibit focus shift between f/4-f/5.6 clearly visible on digital rangefinders. The Hexar design document describes the lens as deliberately under-designed in this regard to keep the size down. On a Hexar the camera would automatically compensate the focus distance at mid aperture (i.e. at f/4 the lens would actually focus at 20 meters if the rangefinder reads infinity) to give optimal result, but such mechanism is not available in the standalone versions. So beware when you shoot distant objects. Also mentioned by others the barrel distortion is fairly obvious.
But it surely is a lovely lens, exquisitely made and weighs under 100 grams. I'm on my 2nd one now. On film it usually does a good job at all apertures since you can't pixel peep. With digital I add 0.3mm (aluminum tape) to the shim (LTM adatper) and the shift looks somehow less pronounced, at least with my sample.
The M-Hexanon 35/2 is unique among 35mm M mount lenses. It's a near-symmetrical retrofocus design very similar to the M-Hexanon 28/2.8, which in itself is almost identical to the Leica Elmarit-M 28/2.8 pre-ASPH. This means it's a flat and highly corrected lens. Not too contrasty since it handles everything in a very balanced way. No focus shift at all apertures. There isn't really any hard flaws except perhaps some samples would not focus correctly on a digital Leica out of box (the Konica-Leica incompatibility myth is partially true). The cure is easy, since the shim adjustment is a simple operation that could be done by any reasonably competent technician. You could even DIY since the M-Hexanon lenses are designed to be adjustment-friendly.
It's large, but isn't quite that
heavy. At 255 grams it's on the same weight level as the Leica 35/2 ASPH and even a bit shorter than the ZM 35/2. The build quality is like what people said - equals to if not better than contemporary Leicas. Ergonomics is perfect. Its fat barrel pairs with the M240/262 brick particularly well...