For me, I really like it. However, I know what brands fit weird, and what shape I am and I can usually tell from looking at photos if something has a weird shape for my body type.
That said, I only shop at places that have a good return/exchange policy.
For example, for shoes, I only buy online from Zappos or Nordies, because I know I can return them if the shoes run weird sizing wise. I always always read reviews too and listen to what people say on there, along with looking at the size charts for each brand if they have them.
Clothes, I only buy from Old Navy because I've been in their stores enough to know what fits me and how it fits me, and Forever 21 is okay if it's just tank tops and stuff like that. Pants, I only buy from Old Navy or NY and Co because I know how those fit.