Nearly half of Windows 7 installs are using 64-bit, despite the fact that the software industry insists on making 32-bit code. Figures just out from Microsoft show that 46 percent are using 64-bit versions of the operating system. Vista only ever managed 11 percent.
For the great unwashed there is little point using 64-bit. They are not going to benefit from being able to use more RAM. But there are other advantages 64-bit Windows can make systems such as ASLR stronger. However despite the fact that Windows 64-bit adoption has more or less happened. Software makers are still dragging their feet and providing viable code.
One problem are Web browsers and their plugins. Plugins like Flash have no 64-bit version. And although Internet Explorer has had a 64-bit version for many years, other Windows browsers have not yet followed suit. Firefox 4 may include a 64-bit Windows version but that would be pants if you can't fun Flash on it, just ask an Apple user.
However it is worth pointing out that 64-bit migration is happening faster than the switch from 16- to 32-bit software. Intel's first 32-bit processor was released in 1985; it wasn't until Windows 95 that anyone used it. It was not until XP came out that 16-bit was dumped completely.