While the Nike+ kit serves a means to an end for many runners, it may not suit everybody's needs. There's the additional cost of the transmitter and receiver (USD $30) for iPod nano owners -- iPhone and iPod touch owners need only purchase the transmitter for USD $19. Then there's the cost of Nike running shoes, which are the only shoes that accommodate the Nike+ transmitter; or as a workaround, you could get a pouch that fits the transmitter on your shoelace; and these usually run about $10.
And then there's the issue of accuracy. I personally find a calibrated Nike+ kit very accurate. For instance, during a 4 mile run (I mapped out the trail on walkjogrun.net prior to running) RunKeeper Pro showed that I ran 3.95 miles while Nike+ showed 3.90 miles. Others, however, have not been as fortunate. And this is understandable, as the kit works by detecting the steps a person takes; and as runners' gaits and strides can sometimes change during a run, this may affect the kit's accuracy.
Accuracy issues aside, RunKeeper Pro, at face value, appears little different than Nike+: at its core, RunKeeper Pro aims to track your running progress, albeit with different technology than Nike+. But dive a little deeper, and what you'll find is a training gem, especially for interval-based workouts.