Is Map My Run Accurate?

Are you familiar with the Map My Run website?  (http://www.mapmyrun.com/)

If not, it's worth checking out.  It's a great (and free) tool for finding runs or mapping out runs you've done to determine their distance. I believe there's also a similar version for biking.

I like to go out on semi-unplanned runs which means I just go, keep track of time, but have no real idea of where i'm going or what the distance is.  When I do this I usually come home and clock it out on MMR.  Thing is, someone once told me that it wasn't accurate.

Was wondering if you've used this site and what your experience has been.  Should I be trusting this information or do I need to take the time out to drive my route and clock it on the odometer?

Input greatly appreciated.