I've purchased generic red plastic cups before, and the experience was terrible. They were hard to pull apart, weren't stable laterally (i.e. the force when you grip it with your hand), and sagged after an hour of being full of liquid. The experience made me appreciate how well designed solo cups are, especially for how simple they look.

I added it to my short list of items I never buy generic (paper towels, paint, and car tires).