I went earlier this year, but I didn't stay long.  I'd love to go back again just to walk up and down Las Ramblas a couple more times.  It's kind of hard to explain but it's the most entertaining street I have ever seen.  While you're there head up to the top of the Monument a Colon (I think that's what it's called), which is at the end of Las Ramblas, for a very nice view of the city.

Edit: Here are a couple of pictures I took while there (they're big so I'll just post links).

The Monument a Colon

Not very well stitched together 360 degree view from the top.