Microsoft Windows has had a 64-bit version since they first brought out a 64-bit version of Windows XP. The 64-bit version of Windows XP was not well supported by many programs, but started the progress of conversion of Windows to 64-bit. One of the main things that 64-bit enables is addressing greater than 4Gb of memory. In the last year or two, it is finally getting to the point where low-end machines come with more than 4Gb of memory. 64-bit also helps with memory layout randomization. 64-bit increases the number of possible locations that memory can be allocated to, making it harder to complete a successful buffer overflow attack. The other thing that 64-bit version allows is various new CPU instructions. These can help improve performance for various tasks.
64-bit drawbacks
One of the drawbacks is that you need 64-bit drivers for all your hardware. At this point though, this is unlikely to be an issue as all new hardware will come with 64-bit drivers. For very old hardware though, there may be no 64-bit driver and make it so the old piece of hardware has to be replaced. 64-bit apps also generally require more memory. This is because to store a memory address you need to use a 64-bit pointer instead of a 32-bit pointer. Finally, although 64-bit versions of Windows support running 32-bit programs, some programs that support plugins need to have plugins that are the same as the parent app.
64-bit Windows Programs
Waterfox is a 64-bit version of Firefox. My usage pattern definitely benefits from the 64-bit version. I quite often have 150+ Tabs open and have seen memory usage up to 6.5Gb.
7-Zip has a 64-bit version of the program. This would benefit from the new 64-bit instructions and the ability to more easily handle larger than 2Gb zip files.
I have been running the 64-bit version of Microsoft office since 2010. Compatibility with other programs can be a big issue with office, as the programs need to be 64-bit as well. This can be an issue for many older programs that have not been updated for a long time. Even many current versions of programs still don’t have 64-bit versions. The only way to deal with that is to remove the 64-bit version of Office and install the 32-bit version of Microsoft Office.
The biggest issues are programs that use Microsoft Word for Mail Merging, Outlook for setting emails and programs that need to be able to read Microsoft Access .accdb files. One of the issues that the 64-bit office version has caused me is that it makes it very difficult to develop in Delphi and use the latest the .accdb file format. This is because the Delphi IDE is 32-bit and the driver to read the .accdb file is 64-bit. This means that while you can develop a 64-bit version of an app that uses the driver, you can use all the RAD environment in Delphi.