Well, Gamestop has definitely driven the price of used games up much higher than before they were a huge success. Prices were already at the break point for me, back then, and now they're just much, much higher.
My issue with GS is pretty simple, I've seen better, in almost every aspect. I had a Rhino Video Games that was in my area, before GS bought them out, and the story hired knowledgable employees who were fair, didn't push their bias, got to know their regulars, and actually recommended things they thought customers would like, rather than what the company was pushing. This wasn't just what they did for me, it's what they did for everyone, and they were very successful.
I go to GameStop, and employees are rude, pushy, recommend useless or stupid things, and it's just not much to compare.
Also, prices are higher, but I pointed that out already.