I love the simple and geeky digital watches by Casio and the F-91W is one of my favorites. I have been wearing the F-91W for close to 15 years and I have gone through 3 of them.
Small: This is one of the smallest watches available. I have a small wrist and this is the one of the few watches which fits well.
Lightweight: This watches weighs like a feather. I can wear this watch for hours and not notice that it is even there. This also makes this my preferred watch if I am running, exercising or hiking.
Operation: The watch is simple to operate: it only has time, date, alarm, stopwatch and a light. Unlike the more complicated G-Shock watches by Casio (which too I have), changing date-time in this watch is very easy to figure out by pressing the buttons.
Battery: The battery lasts 5-7 years. In fact, I recall changing the battery only once on my F-91W watches. This is because the strap or the watch usually wear out much before that. And since the watch is inexpensive, it is easier to just replace the watch itself.
Strap: The strap is made of resin and is super light and soft. This is the only strap I have found which actually has holes that could fit wrists smaller than mine! This strap is the watch’s weakest point too - it wears out easily and breaks.
Price: At just $20, this watch is a no-brainer to get. Due to the strap breaking and general damage, I end up buying a new F-91W when the battery wears out.
Accuracy: Digital watches are generally more accurate than the mechanical watches. I found that my F-91W runs about 20 seconds faster per month. I use this watch during work day, meaning I rely on it to join work meetings, so this difference of a few seconds per month does affect me. Watch folks consider this much accuracy to be good enough, but I personally wish watches were more accurate - like a few seconds in a year. I have to end up fixing the time on the watch every few months, once it has deviated by a minute or so. I have searched and not found a solution for this - apparently all digital or analog watches are this inaccurate. Casio has watches that can sync to the time signals transmitted in US and UK from atomic clocks - but their syncing mechanism seems to be troublesome according to the reviews.
Repairs and replacements: The strap wears out or breaks easily if you use the watch for running or exercise. Replacement straps of all types are available easily on AliExpress. I have tried nato straps, but have come back to the simplicity of the original strap. However, all replacement straps seem to use a metal clasp instead of the plastic used in the original. I find this irritating since the metal bites into my skin.
In summary, I find this watch to be a must-have for daily wear at home and for running or exercises due to its light weight and small size. I prefer to wear other Casio G-Shock watches for more formal occasions.