I bought one of these things 6 months ago as I had registered for the great north run and was starting running on a regular basis
as a motivator, i find it excellent, i have always found running fairly soulless so the idea of logging up a number of runs and being able to view them graphically on a website works really well for me. it is free to register with the nike plus website, i don't have the fancy trainers, i use a pouch- and itunes does this automatically- you need at least a second generation nano, which was a bit disappointing but after that small hurdle i was sorted
one small point, i know how far i run- because i plan my runs on google maps but the accuracy is fairly sketchy sometimes- i believe it uses an accelerometer (sp?) so if you slow down or speed up, or go up or down hills the distance is way off, and yes i have calibrated it a few times. the alternative is a gps system but these are hundreds of pounds and at the start of my running i really didn't know if i would ever get into it so went for the cheap and easily good enough option
definitely worth a pop, get out there and run!