Hi
When you say Crossfire do you mean ...
DX11 Crossfire.
or
DX12 MultiGPU.
I use both initially with older HD7970/R9280x combination since 2014.
In 2017-2018 I was running R9 Nanos / R9 FuryX/R9Nano, and R9 Fury X in Crossfire.
RE: What has your experience been with it?
Good on titles that supported it.
Terrible on some titles -by terrible I mean very bad visual flickering in some titles.
RE: Has it been a hassle free experience at all?
No it is definitely not a "hassle free experience".
You really have to understand when it should be used and you need to know there is additional CPU Overhead.involved in running it.
You need to make sure there is enough PCIe Bandwidth on the motherboard slots you use.
Some older game titles can run ok in Crossfire with low PCIe bandwidth and or mismatched PCIe slot bandwidths.
Others have problems with that.
I see cases where a game will launch fine flicker free in fullscreen mode but if I alt tab over to an application then back to the game the screen flicker will be awful.
Sometimes if you cam manage to Alt Enter between Windowed and full screen mode you can get rid of the flicker. Other times you have to restart the game.
Some post processing effects and Nvidia Gameworks option cause problems when turned on in some games running Crossfire/MultiGPU.
BF1 is a good example of that.
Also if CPU utilization is at ~ 80-90% when running DX11 Crossfire then flickering happens on some titles like Crysis 3.
RE: How big of a difference does it make on performance while gaming?
If it works well, the FPS scaling I see with my GPU/system ranges from 1.6-1.85 on games I tested recently - this will generally be on demanding games at high resolution and game settings (2K/4K Ultra BF1 for example).
RE: Have games been optimised well for it?
Some specific games, especially some AAA titles that were launched around the time and after AMD released the RX480 on 29 June 2016 run really well for me.
I believe that AMD did put lots of work into DX11 Crossfire and DX12 MultiGPU performance at that time because it was the only way to compete with a single GTX1080.
In other cases DX11 Crossfire or DX12 MultiGPU support may be initially promised by game developer but may not work properly at all for various reasons.
Very few users run DX11 Crossfire or DX12 MultiGPU and very few DX12 titles so I think it is low priority for the game developer. and when RX Vega 64 was launched, DX11 Crossfire was 'de-emphasized'.
RE: I'm thinking of buying two GPU's when I buy my brand new PC and one very high end CPU.
If I were you I would spend your money purchasing a single high end GPU.
I think you should only use more than one AMD GPU if you need them for OpenCL Compute or Blender Rendering.
Remember more than one GPU will push up your PSU needs.
I run single RX Vega 64 Liquid and it reports ~ 380 Watts GPU power used running BF1 at 4K Ultra.
Also if you are looking to pick up 2 RX Vega 64 AIB GPU's look at the physical size of the cards that are worth buying. - Sapphire Vega 64 Nitro+ or PowerColor Vega 64 Red Devil.
They are 2.5->3 slot wide and the Sapphire weighs1.6 Kg requiring a GPU support bracket.
You may have severe problem fitting them in a case and getting enough airflow to the GPU coolers.
Options are a pair of Vega 64 Liquids or a pair of reference cards on waterblocks.
Other reason to try DX11 Crossfire or DX12 MultiGPU is if you get the chance to purchase a compatible GPU at a very low cost and you are interested to try it out.
If you search the forum you will see examples of me testing DX11 Crossfire or DX12 MultiGPU.
It is not all bad news, but there are serious downsides.
Here is one good example of what is possible: BF1 run at 4K Ultra settings with R9 FuryX and R9 Nano in DX12 MultiGPU. - YouTube
That runs ~ as well / better than a single RX Vega 64 Liquid on my PC.
A pair of R9 Fury X's run even better.
Bye.
Ok, thanks for the info. I think I'll just buy a single high end GPU and be done with it. I was only contemplating getting two GPU's for gaming, and it doesn't sound like its worth the hassle that comes with it. I would buy the new Radeon 7, but I'll try and hold off a little longer for more news on the next Navi GPU later this year. 
