Edit: Of course, what I'm really interested in is "Is the idea that it takes 10000 hours to master a skill set true in enough circumstances to make it a useful guideline?"

The '10000' hours approximation seems surprisingly well founded, based on the research that Ericsson et. al. reviewed in their works. Obviously this is to obtain 'expert' level performance and you can still get 'good enough' levels from far less time. Also note that they specify that many of the hours must be deliberate practice and not just performance.