Some people?
I guess the best way to put it is that 6.5 has a much different 'feel' than 2006, and that's going to cause lots of complaints. However, I think it's fair to say that 6.5 was a much more stable game, and one that seemed to handle multi-player games better.
This is a pretty controversial subject (to say the least), so I think even this modest comparison will be debated and analyzed. The best way to understand the debate would be to play both games yourself.