[Mate 8 General] beachletter:
There are plenty of EMUI themes on xda that gives you material design icons , you can just install one or use together with 3rd party launcher like Nova, no need to root or switch rom for this.

The notification and quick settings menu looked nothing like ios, unless you think anything with some transparency effect equals ios. To me they look much better than that bluish-green and busy looking drop-down on touchwiz, although touchwiz does give you a bit more options on what to put on the quick settings panel. The (non-quick) settings menu may have more of a resemblance to ios in UI design, but so what, it looks clean and easy to use. Besides, how often will you be looking at the settings menu?

Overall, I'm generally satisfied with the aesthetics of EMUI 4 (I'd argue some parts better looking than touchwiz or even stock android). The main things I don't like are the stock icons and forced icon boarder, and to a lesser extent the default launcher. These are easily solvable by installing a theme and Nova Prime.

As far as user experience goes, the system runs stable, fast, and not buggy for me (what's with the hating that made you so sure Touchwiz optimized better? ;) ) But there are some things you may want know if you haven't used EMUI before:

By default there will be power-intensive prompts and it will auto-close apps while screen is off. Some people find it unnecessary and annoying, but they can be turned off altogether or be excluded on selected apps. Just adjust it to the way you like in settings.

Some people complained about missing or delayed push notification when the phone has been idle for a few hours, I experienced it as well on early ROM versions, but I no longer encounter this problem since the last few OTA updates, push notifications now come like normal (instantly).

If you rely on on-going notification apps that shows dynamic interactive icon on the top status bar (e.g. an always-on weather app that shows current temperature in its icon), it may not work on the EMUI status bar as it will only show the default app icon. I don't use these kind of things so I'm not affected, but that's just me.

EMUI 4.x has extensive app specific battery-use management, network-use management, and phone/text harassment filter, which are great in general. But I find the harassment filter a little bit lacking in that you can't block text and phone calls using separate rules.

The camera hardware is just meh/passable if compared to other high-end phones, but still better than mid-range phones like the Galaxy a9. The camera app on EMUI 4.0 is good, on EMUI 4.1 it is great (the best I have used). EMUI 4.1 is recently being pushed to some Mate 8.

I don't know if you used 6 inch phones before or not, but I would think that having a back fingerprint scanner rather than a front one is a blessing and not a con on 6 inch devices. I'm a fan of front scanners on smaller phones, but if you tried to hold a 6-inch phone you will know how much more comfortable it is to unlock with your index-finger than with your thumb (unless you have huge hands, or that you use your phone placed on a table much more often than holding it)

If you are a die hard stock/MD fans and can't stand any EMUI design cues, or if you like flashing different roms, then you will have a hard time owning an Mate 8, because it is unlikely that you will find Cyanogen/AOSP rom for this device given that it uses Kirin SoC.

But if you don't like manually flashing roms, the Mate 8 will actually receive better rom support than the a9, because Mate 8 is a much more important product line for Huawei than the a9 is for Samsung. The Mate 8 has been on android marshmallow since the beginning, yet many Galaxy a9 are still on lollipop today (marshmallow for a9 just rolling out for a couple weeks). Mate 8 will likely receive support up to Android N/EMUI 5.0, but for the a9 I'm less confident about it.