You don't download anything. All you do is go to your game folder and double click on eso64.exe to open up the game (it won't open the launcher). Tada. You will be using 64 bit client

I'm not that technical, so in theory (my theory, of what I've heard) it should allow the game to use more of your system memory. Be warned though, the 64 bit client has been a bit buggy hence why it is not auto-launched from the launcher. I see they did some fixes today though, so I'll definitely be trying it out again!

I'm using the 64-bit client and it works really well for me as compared to launching the game through the launcher. Guess it's because of me running a 64-bit OS? Dunno exactly.

I've not compared the recent 32 bit client to the 64 bit client, but I have been using the 64 bit client exclusively since it became available. I've noticed a slight improvement to texture quality but I think this might be more to do with DX11 support, which I assume the current 32 bit client also has.

I've lost 5-10fps on average since the old 32 bit client and there are some distracting bugs such as occasional pink textures and erroneous sounds which can get really annoying. There are also intermittent fps drops which can be noticed through the mouse and screen, this is on top of the general fps drop which can't be noticed through the screen. This intermittent fps drop can happen in pve or pvp and seems to happen when the erroneous sounds are at their worst. Yesterday I was in wrothgar with my dc alt and getting really bad fps drops and sounds, logged out and logged back in on my ad main who was also in wrothgar and no issues whatsoever.

Basically there are a load of bugs but the only ones I can confirm are exclusive to the 64 bit client are the pink textures and erroneous sounds. The frame drops may well be happening on the 32 bit client too. Any pink textures can be fixed with a client restart, not sure if that also fixes the sounds; I've just been tolerating them unless they get really bad. Yesterday in old orsinium, on my dc alt, somehow the sound effect of boundless storm activation got replaced with the sound of harpies' death throes. So every time I activated boundless storm I heard a harpie dying. Was pretty amusing tbh.

TL;DR: I havent noticed any benefit that I can definitely attribute to the 64-bit client, only bugs, but I continue to use it all the same