Linux on amd64
You'll probably find that linux is probably the OS with the most native amd64 (x86_64) users. BSD variants and Windows mostly have poor to no amd64 support and/or a smaller user base any way. However, there are still a few drawbacks compared to linux on 32-bit x86.
Drawbacks ? Where ? Why ?
Nowadays, there is a lot of software linux users are accustomed to use that is available in binary form for the intel x86 platform only. There are big commercial applications like VMWare or games like Doom III. But there are also small freeware pieces of software, the most common probably being Java and the Macromedia Flash plugin. Using x86-only plugins on amd64 linux natively is impossible since the web browser and plugin must link directly. There are workarounds forkonqueror, but not everyone wants to use konqueror for every day web browsing. Many people also like watching videos in new wmv formats, which is currently only possible with the binary-only w32codecs. Some open source programs do not work in 64-bit space either; the most notable being OpenOffice.org.
So why do people use it then ?
One word: speed. People like it and linux applications can often give you more of it when they are given 64-bit memory access - more even when, as usual in gentoo amd64, they are optimized for the specific processor you are running. Since the amd64 processor can run 32bit x86 code, there are also workarounds that allow you to run all the applications above on amd64 linux, although sometimes with minor tradeoffs. In the setup described below, I can run firefox flash.site.com and flash.site.com comes up in a x86 flash-capable firefox. This article is ment to explain how I did it.
So, how ?