Great Sales People.
It's true that you can find more brands and selections elsewhere, but BtPO has more Development than anywhere else, and the sales staff is refreshingly kind and helpful without being pretentious. I think these two factors alone make it worth a visit.
I came to BPO to get a pair of the famous 7 jeans. The staff was so helpful. They worked with me until I found the perfect pair. Their prices were better than other stores, and they even hemmed my jeans for free!! The rest of the clothes are very cute and hard to resist. I love thier Development stuff and great jewelry too. They are freindly, and create a really comfortable place to shop. It's small, but all cute things so you don't get overwhelmed. Behind The Post Office is my vote for favorite SF store!!!!
Interesting but small selection.
I enjoy stopping into this boutique just to see what they have in stock at any given time. They sell mostly trendy clothes that edge toward classic style. It's a small place, and I rarely actually make a purchase there, but the selection is always interesting and high-quality. They carry both men and women's clothing, as well (usually) as a few shoes and accessories.