Ian
I've not used the Casio Pro trek but have a Suunto Vector (which cost me £115). The Vector is very accurate, providing you remember to reset the altimeter when at a known height, this is especially true when the weather is changing as obviously the barometric pressure will change too. The compass is Ok though uses a lot of power. As far as the thermometer goes, the watch is attached to your wrist so the normal reading you tend to get is the temperature of... yep you guessed it... your wrist! The temperature sensor is normally use to compensate the barometric measurement for increased accuracy in altitude (check the Casio does this). On the subject of wrists, make sure the strap is large enough to fasten over your waterproof, mid layers, etc, as you don't want to be faffing around removing gloves and rolling up your sleeves in the middle of a Scottish blizzard.
They tend to be more useful on big pointy hills and less use on plateau style hills. If the Casio has a "Rate of Ascent/Descent" function it can be very useful for calculating your ETA to the valley floor or summit.
Oh and the usual disclaimer.. It's a gadget and you should not totally rely on it, you must be able to navigate without the aid of any artificial devices. So that's on a hill, naked, using nothing but the position of the Sun and Moon and the migratory passage of wild animals. (I think I'd hide a GPS up my .....) :-)
