I spoke to someone not long ago about this - I've taken my skateboard with me the last couple of times I've gone to Ibiza. The roads aren't the best but it was really cool skating along the prom bit from the Bay to the Harbour.
I did tend to only go out in the evening though, when its not as busy as during the day and before the crowds come out at night - as I know how annoying it can be, having skaters weaving in and out as you walk along...
Plus its far too hot during the day anyway...
But rollerblades would tale up a lot of room in your luggage - my board is ok as it just takes 5 minutes to take the trucks off and put them back on at the other end...
Though, if you can find somewhere to hire them in once you are over there, I'd say go for it - I had great fun and would recommend it
