It's nowhere close to twice as fast, and in certain use cases can actually be slower, and many system extensions and third-party programs will either not work or may have issues, so it's not recommended for just about anyone. I think the fact it's not enabled by default, and in fact requires a bit of research and a bit of a hack to even run in the 64-bit kernel by default should be a tip-off that it's probably not for the average user.
In other words, you're not likely to see much (if any) benefit, and it's very possible it could cause issues for you, so don't try unless you find that exciting. I personally prefer my machine to work, so I run the 32-bit kernel for now.
jW