I’ve had mostly good experiences. I have ordered tons of stuff from Modcloth (like half of my wardrobe). The tricky part is the sizing, but luckily they have a good return/exchange policy. I find that most of the clothing they sell runs small. I pay a lot of attention to the reviews, which usually help me figure out the sizing. You can enter your measurements and it will show you reviews from people with the same/similar sizes.
That said, I bought a dress with the shoes and I couldn’t get it over my boobs, despite the fact that I sized up like the reviews suggested. So it can be a mixed bag.
Overall, it’s (by far) my favorite online clothing store. When I return the above mentioned dress, I will buy a different one to replace it. Ha ha.