NOTE 1: I am not associated with Sofirn in any way. I contacted Sofirn to ask if they would be interested in making an improved keychain light design based on the C01S. Currently, it can be difficult to recommend existing keychain lights due to concerns about quality, design choices, or high prices. Happily, Barry0892 indicated that Sofirn has such plans, which motivated this thread to gauge interest and get BLF involvement.
NOTE 2: Naturally, opinions and needs will vary, so I think it is crucial to have a clear focus on what are the goals of a given design, and perhaps more importantly, what are not the goals.
The following is my personal philosophy on what makes a good keychain light (subject to change 
).
GOAL: To have a dependable and useful light present for unexpected momentary needs, while being convenient and unnoticeable the rest of the time.
Non-priorities: highest lumens, longest runtimes, ideal color temperature or tint, exotic materials
Description of terms:
- “Useful” means having enough brightness and throw for seeing short to medium range objects (eg. in a closet, basement, under a table, around a car in the dark, a dark alley, etc), and wide enough flood to light up a few meters around and in front of a person when walking in the dark.
- “Dependable” means excellent design and build quality. Usefulness requires solid reliability—what good is a light whose head fell off, is flakey when turned on, or self-discharged itself two weeks ago?
- “Unnoticeable” means a strong focus on small size and low weight, which will allow it to be “present” at all times on a keychain without giving it a second thought or noticing that it’s there.
- “Present” also means it must be low cost so that it is affordable to be carried as a handy tool that can be beaten up on a keychain, not an expensive showpiece item that one would worry about and leave in storage.
- “Convenient” means USB-rechargeable so that it can be charged almost anywhere (even city buses have USB charging ports), and spare batteries, chargers, and knowledge of lithium ion battery protocols are not needed. Muggle-friendly.
- “Unexpected momentary needs” means that long battery runtime is not important especially at high brightness. These are simple, daily life moments where a light would be useful, but for which one might not anticipate to bring a full-featured flashlight.
I fully realize everyone has different goals and priorities, especially on BLF. This will not replace a AAA or 18650 light. The idea here is to have a neat little device that will be great for 80% of people, and which would probably be sufficient in 80% of day-to-day casual needs for light. A well-optimized true EDC for the common folk.