I have the Garmin Forerunner 101 which is slightly large but you can get them further up in the series which are more like a watch. I have found mine invaluable for what I need it for. There is a training partner function which can pit you against a virtual partner and beeps at you when you slack off!! I have found mine is very accurate (to about 15 ft) and they are cheap(ish). I picked mine up from eBay for about Â£60 but you can get a cheaper one if you get lucky!! The link is for a 201 which does everything the 101 does and looks the same but also has the function of uploading your runs to a website so you can track your progress (the 101 will store approx 2 years of data)
If you get the fore runner 301 it comes with a heart monitor.
Should you consider the fore trex series (based on the Geko range but wrist worn) you have the added option of navigation (101, 201) but the fore trex 301 can also be used for fitness as it too has a heart monitor but then able to be used in the field.
Point to note that the 'watch type' gps have lithium batteries and if you are out for extended periods of time (eg excercise) and you can not get to a mains socket to recharge it then it dies and is useless.
However if its fitness then either the forerunner 101/201 or the foreTREX 301.