I just think it's so rude, snobby and nasty. We can't all afford designer and high end clothing, and it doesn't make someone a total scumbag if they choose to/have to wear cheaper clothes.
DH's friend and his wife both wear designer and high end high street clothes, and dress their 3 year old daughter in the same way. Fair enough, that's their choice and they all always look nice. However it irritates me that they look down on others who don't dress as they do (such as me, DH and our kids!). It's little things like asking where an item DD is wearing is from and if we reply it's from Asda/Next/Tesco they pull a bit of a face and say how they would simply never shop there. They also make a big deal about how they would never shop in Primark as they think it's horrible and that they'd hate anyone to see them going in or coming out of there. They were also once totally disgusted when I said DS's trousers were from a charity shop, and one of them made a jokey gagging noise.
I also used to work with a woman a few years ago who referred to lots of high street shops as "common" and "disgusting" and made a huge thing about only shopping at places like Reiss, Selfridges, Karen Millen etc. She always talked about how great her wardrobe was and how she could never bring herself to shop in the shops that the rest of us shopped in.
These kinds of attitudes really bug me. It's just so nasty and unnecessary. By all means buy clothes wherever you want to and can afford to, but don't assume that anyone that doesn't go to the same places it beneath you!