It's not really bad. It depends on what you are doing with the computer. Photographs and movies will look just fine (in fact really really good). You will notice it with regular desktop apps, but I don't think most people will find it disturbing (or even notice it). But for those who do design, illustration, layout etc. it is disturbing.
I can't detect this problem on mine. I've tried various shades of white/gray and they all look consistent across the screen with the exception of some slight dimming in the corners. I've also tried dragging a few documents and photos around to see if I could notice any tinting from side-to-side or top-to-bottom. Same result. No color shifting/tinting.
Unless you've spoken to every single iMac purchaser, I doubt you could say it is a "common issue." A problem (i.e. display issue) is going to look more common than it really is because those with issues are far more likely to post about it.