Quote Originally Posted by SadieKate
One thing though is that it seems to over estimate the total climbing. I had a very short ride that came out high but I finally put it to the test this last weekend with a longer route. Routeslip said 4,134 ft for the route below but my Polar said 3,395. Pretty significant variance.
The way routeslip works is that every time you add it point it determines its altitude and then compares it to the last point looking for a gain. Since it doesn't know anything about what happened inbetween these points it should underestimate both climbing and distance. I don't have any way to check my real climbing but I know my actual miles are always higher than what routeslip says. These underestimates should be greater for routes with many short altitude changes as compared to routes with long sustained climbs since your point picking will rarely hit the actual peaks.

Regardless of underestimates, if you do similar point picking (distance between points) for two routes you should get results that can be compared relatively.