To GPS or not to GPS
This is not a review or endorsement of a specific model or brand. For the record, I've owned a Garmin Forerunner 305 and 405. The 305 is super easy to use with a big data display. The only downside is the unit is huge. It was like wearing a Rubic's Cube on my wrist. Eventually the irreplaceable Lithium-ion battery died and I switched to the 405. This had a form factor much closer to a regular sports watch albeit a smaller display which meant showing less data. The menu navigation wasn't as intuitive as its predecessor and this model used an iPod like bezel which works great provided their is no moisture present (make sure you don't run in the rain or sweat too much).
The biggest advantage of owning a GPS enabled watch is near perfect information telemetrics on time, distance, pace, hear rate, elevation, etc. sliced any way want but during and after your run. This type of information, level of detail and degree of accuracy are critical for anyone wanting to get scientific and analytical on your training. Another plus for GPS watches is freedom. Gone are the days of getting in your car to measure out loops and look at a map to estimate distance. Simply head out the door and go in any direction and know exactly how far and how fast you've gone. This is great for anyone looking to explore a new area or traveling. While heart rate monitors are nothing new, the hear rate functionality is an important feature for anyone looking to optimize their training as heart rate is a much better predictor of effort than pace or how you think you feel.
GPS watches have a few downsides. The first is cost. Most models run several hundred dollars many times the cost of a simple sports watch. Another downside is the time it takes to find a satellite signal which is required before every use. Most days this isn't a big deal but starting in a new location or being near mountains or tall buildings can lead to long times for finding a satellite. If you're pressed for time and only have a hour to devote to running, waiting 15 minutes to find a satellite signal is not a good use of your limited time (and quite frustrating, not a good way to start your run).
Overall, I think a GPS watch is a must for anyone looking to get serious and boost their performance. As the technology evolves GPS accuracy has improved and form factors are getting to the point there is little distinction between GPS and non-GPS sports watches. They aren't cheap but if you consider the tremendous benefit they can provide if used properly, this is money well spent.