I like having a watchband compass. It's there if I have no other landnav gear or even (gasp!) a PSK, and when hiking/whatever its there as a backup. It's also convenient to be able to quickly look at the wrist to verify general direction, quickly orient the map, etc. without retrieving the main compass.

I wear a Suunto Clipper on my watchband every day. IMO the Clipper is superior to any similarly sized button compass. For a tin-based PSK I would just throw a Clipper in there.

I also have the Suunto M9 wrist compass. This little unit is surprisingly accurate due to its gunsight style sighting system. For testing I have gone into the field with the M9, a Silva Type 54 prismatic, and a Suunto MC2-Global mirrored compass to compare azimuth accuracy. I found that I could sight with the M9 to within a couple degrees of the bigger compasses. I used a GPS with known waypoints in an area for which I have very accurate maps as a control/comparison.

One caution: the M9 has a strong magnet and with it on your wrist, it will affect the bigger compass in your hand. During my testing I noticed that the readings taken with the big compasses were different than readings with the M9. Then I realized that when sighting with the M9, no other compass was nearby to it but when holding the other compasses the M9 was on my wrist, a few inches away. When I removed the M9 the readings of all the compasses jived.

I did not see the same behavior with the Clipper so I believe it has a less powerful magnet.