Whistles are very good to have. You can exhaust yourself shouting for help and still have a fraction of the effective range and duration of using even a mediocre whistle. Once in your teeth a whistle doesn't need hands to use and takes just a little more energy than breathing.
They are also cheap, light and fairly compact.
This later specification is what has guided my selection of whistles. I like the Fox-40. It is loud with tone that stands out well and is easily locatable in most situations. The one problem I have with the Fox-40 is that it is a bit bulky. It doesn't disappear in a pocket or hung around the neck.
The bulkiness is annoying, ideal survival equipment disappears until needed, and causes me to not want to carry it or leave it behind. For whistles it is something of a minor health issue.
Try this: Put a Fox-40 in your pocket or around your neck hung on a string and fall on it. How does it feel? Did it leave a bruise? How would it feel if you were slammed and bruised like this every hour or two? Possibly for days? How do you think it will effect your ability to sleep, function or love life? How would this effect your ability to survive?
There is an old saying: 'It isn't the mountains that wear you down. It is the pebble in your boot'. Minor annoyances wear on patience and drain the endurance.
This small consideration, bulk as annoyance and source of injury, led me away form bulky whistles. An alternative I found I like:
http://www.bestglide.com/nato_distress_whistle.htmlIt is not quite as loud, in my estimation, but being flat (easier to land on and keep smiling), dirt cheap and in a bright color has advantages.