I think Windows 7 will be much better than Vista, but apart from slimming it down and making the Apps more Exzchange aware I have not seen a big difference in 10.6. As Fryke mentioned unless the final release changes it will not run on PowerPC based systems which may well upset a lot of users.
Unfortunately with the world economies going as they are I fear Apple may be the biggest loser in terms of OS market share, it might be the best but Apple hardware is the most expensive and that will hit them hard. I bought my Macbook almost a year ago, but if I had to replace it now I would find it hard to justify the price tag much as I love it.
Most popular OS for the next couple of years, got to be Linux as it has the best price point, Free and it runs on both PC and Mac hardware.