http://www.urbandictionary.com/defin...=rollerblading 
For Cardio I can get into similar heart rate training zones compared to biking and running. Mountain biking 120-170 range Running 130-180 range and "rollerblading" 130-170 so the drop off isn't big. Factor in that it is low impact and I have yet to be injured rollerblading it seems to be a good way to cross train. Also builds muscle groups that are hard to hit and will make you a better runner/biker as a result.
I can't deny being out on the paths and having people give you the gay look though. It is a very strong stigma but not enough to keep me from hitting the paths. Growing up as a skater this is my excercising on wheels fix and is good for core/cardio/stability/balance etc. and besides it keeps me from being mid age guy on a skateboard
Things from the 90's come back