I have a page of blading sites in Barcelona if you simply want some idea of good places to skate.
If you're looking for fast-paced rollerblading action in Barcelona, there is also a group of us who skate on Tuesday and Thursday nights between approximately 11pm and 2am. There have been as few as 9 people and as many as 85.
At the moment, the standard is fairly high. Very few people have a brake on their blades, and we occasionally go down some quite steep and long hills. There is one that members of the group apparently got up to 63kmh (about 38mph) on. There is no need to be so extreme, one can always swerve to slow down. Also, the group tends to stop and talk (or wait for slower people) quite often, so if you're tending to fall behind, you'll have a chance to catch up. We're often spread out over 4 blocks or so. Sometimes more.
If you're not too confident on blades, particularly if you're not confident about stopping quickly and going down reasonably steep hills (often with traffic at the bottom), this group is probably not for you yet; unless you have a death wish, in which case you should certainly come.
The distance traveled varies between about 10km and 35km (6 to 22 miles), at least that's what I have seen in the last few months. We go all over Barcelona.
If it is raining or drizzling or the streets are wet, no-one will be there. Keep in mind that many of the streets in this area get washed down every night. If they're wet, make sure it was rain that did it before deciding not to come (hint: check other streets, look for men in orange uniforms with big hoses).
For more information on inline skating in Barcelona, including the times and meeting places of the regular weekly outings, please visit www.patinar-bcn.org. If you want to contact a human, try mailing Gustavo Noussan at YETIGUZ -at- terra -dot- es. Please don't contact me, I hardly ever skate any more!