I use my android Sony Xperia to track my runs - it does what I need and tells me how far/fast I'm going, gives me spoken indications of my pace/distance covered/etc and also shuffles my play lists for me...
I'm pretty positive that GPS watches, being specifically designed for the job, ARE more accurate but I think the phone apps are accurate enough for the average runner.
Plus, if one day I'm a bit slower than I expect, I just blame the phone