I've seen websites that screen programmers by their ability to write code. It's a service that you enter a programming question into and then send out a link. Job candidates program their solution to the question as they are timed and recorded. The person who posted the question can then playback a video of their candidate programming the script. This video allows them to see how quickly and neatly their job candidate can code.

Are these types of services worth it? What caveats and hangups are there to using such things to screen potential hires?