We have an iPad 3rd gen, and an iPad Mini. Are the screens different? Of course. Did I know that when I bought it? Yes. Is the screen so bad that it's unusable? Not even close. To some people, the lack of retina screen is a dealbreaker. To others, it's really not an issue. I'm more than happy to have a smaller, lighter device with no retina. I can go from my 15" high-res MBP screen, to my iPhone 4S, to my iPad Mini with no problem at all.
Everyone is different, and different things bother us. No device is perfect for everyone, including the mini. But just because some people don't like it doesn't mean it isn't suitable for a lot of other people.