My problems with runkeeper are that it doesn't seem to poll the gps often enough to stay accurate, and it doesn't keep track of altitude. So while it might be fine for running along city blocks, I've found that for trail running it cuts too many corners, resulting in fairly wide swings in its calculated pace. That and it's lack of altitude info means that over longer distances, it will invariably come up short, in my case, 5% - 10% of the actual distance. MotionGPS is much more accurate in both those respects, but unfortunately it doesn't do voiceover, which is the one nice thing I like about runkeeper.
Then again, running with an iphone is a pita anyway, so I've really only used either app about three times each over the last year. If Nike's heart rate monitor is decent kit, though, I'd consider getting a new nano just for that. Though the polar monitors might still be a better deal if you don't need a new ipod anyway. GPS is cool, but heart rate is far, far more useful a thing to keep on top of from a training standpoint.
