I found Riera Baixa Street a rather boring street to visit,if your on holiday and your into your retail therapy there are some boutique and vintage cloth shops here,they looked busy but of i have no interest at all in shopping while on holiday, that said the architecture here is quite good,some great examples of Gothic style architecture but I wouldn't go out of my way to go to this street.
If your a keen shopper then a street that you should head to but for me I found the street a bit boring.