Hi DP
We just got back from Barcelona about 2 weeks ago and used this service (though there are TWO companies, and I think we might have used the other one). We didn't initially intend to use this, but by the time we got to Barcelona our feet hurt so bad it was just so nice to get on the bus without having to think. One night we just rode it for about an hour, and it was so nice to be up top riding around in the wind after a really hot day of walking around.
Barcelona sites are pretty spread out, which is why I appreciated this service. All metros seem to converge at Place de Catalunya but the metro stop is so big it feels like you are walking 10 minutes underground just trying to find your transfer train.
The only time we waited to get on the bus was at Placa de Catalunya (we waited about 30 minutes the first time we got on the bus, it was a Saturday and just after buying the ticket). The bus comes about every 6 minutes I think? We never really waited. It was also handy to go up to the out of the way sites like the sports areas, which were priorities for my boyfriend to see. Anyways, I'd do it again! It was kind of expensive, but we bought ours from our hotel and it came with coupons like 20% off some of the Gaudi sites. Have fun!