I have both the H503 NW and the red version (H503r?). They are good lights, and even smaller than the H53 model -- without the reflector in the way, ZL is able to move the battery further up into the head, thus shortening the entire light by about 6mm or 1/4".
The quality of the light is very nice -- complete undifferentiated wall of light, no hotspot, no difference in concentration from one side of the beam to the other. If you are working on something at arms-length, there are no artifacts of any kind. If you hang it up on the ceiling, the entire room is evenly illuminated.
However, I have to warn you that there is no throw at all. It's a good light for arm's length work, but forget about seeing something 10 meters away.
The other result of the very wide angle (120 degree) is that it looks like it is putting out fewer lumens than it is, so you will want to turn it up higher than you normally would. 15 lumens in a tight cone can put out a lot of concentration on a target; 15 lumens distributed over almost half of a sphere is losing all of that concentration. Another way to say it: you're lighting up what you want to see, and you're also lighting up a lot of stuff that you did not intend to look at. Yes, fabulous for peripheral vision, but a lot of your light is going where you are not using it in any given glance.
Short answer -- they are beautiful mules, and if you know what it's like to work with a mule, then you will know what you are getting into. For working around a campsite at night, it's going to be a good light. For walking at night, less so. For spotting what's beyond that tree, hopeless.
(Why do I have the red version? Great for bike-lights, because you *want* to be seen from a very wide angle, the wider the better)