I have been cycling and running for the last 15 years or so, and during this period the body has adapted to the training load. In the beginning I was making huge progress. Sometimes I could see improvements from one training session to another. Later it got harder to take it to the next level, and I was sometimes happy just to maintain my racing form from one season to the next.
With PowerCranks this feeling of imidiate response to the training is found again. I know the improvements will be less noticable over time, but so far I have only gotten better each time on them. 

Now after almost 2 weeks riding PC´s I´m able to ride for 15 minutes straight without a break and I can do that about 3 times. That is on the home trainer, and I think that doing close to 2 hrs on the road is possible.

I haven´t been able to see any progress on my running so far due to a knee injury, but I hope that riding the PowerCranks make my recovery a bit easier. Since not being able to run I´m also happy  to have another thing to concentrate about and challenge me in every training session.