I have had a garmin for 3 years now, just a 205, the bulky cheapish one, but it has multisport, gps and easily programmable intervals. However on a recent spur of the moment holiday i forgot to pack it - i love running in new places and uploading the maps when i get home, so much so i decided to buy a GPS watch at the airport. The only thing available was the Nike+ GPS.
So I have both (although the nike for monly a few weeks now)
Nike+ ( In comparison to 205)
Pros - great fit on the wrist (can pull layers on/off easily), better/faster GPS pick-up (than the 205 anyway), friendly interface online, vibrate function would be good for races, good to have the option for a foot-pod for heavily built-up cities/treadmills,
Cons - limited information available on screen (only 2 data fields to garmins 4), touch screen is not particularly handy when running, battery life is very poor as it continues to work as a watch when not used for running, use for speed work is very limited
I think the Nike will be my back-up watch, I'll always be a Garmin girl...