.NET install isn't required, skip or cancel the install. Works perfectly fine other than the .NET install box coming up when you press play

No matter how I try to start this game, it hangs at "Updating executable" and then crashes.

Works out of the box perfectly. Played campaign coop only.

Slowdown can happen if you checked SSAO. Without it game runs 30% faster. Without SSAO you can expect stable 60fps on 12tdp.

if you can get through the mixed reception and enjoy the game, playing it here can be a great time. otherwise dont bother playing at all.
tabbing out and having system popups can bring you out of game and sometimes lock you out, requiring a restart at least on my hardware you can exepect that
restarting sometimes lost me my Graphics settings and i had to reset them to make the game playable on my hardware

Terrible game, but it installs and runs with no issue at all
Install, press play - flawless.

Game ran perfectly ootb. No tinkering required.
Works out of the box.
Just plain worked.
Works perfectly fine. No issues here
Flickering in the menu, but unticking the AA solves the issue.
Small screen makes the game less ugly. You can run it on max setting with stable FPS. Untick the anti-aliasing and you are good to go. Controls seems fine. Multiplayer pretty much dead.
Plays without issues
gamemoderun strangle --fps 143 --vsync 3 %command%
MaxSmoothedFrameRate=144 in PecanEngine.ini to bump max framerate from 60 to 144 FPS.
Act I of Stasis Interrupted DLC kept crashing at certain points, but setting quality to low helped circumvent those crashes. All of the other levels were completely fine.
Co-op worked flawlessly.
The game works out of the box, there's no real need for any tinkering. If you play at higher refresh rates, modify the corresponding line in PecanEngine.ini.

When start the game first time is necessary to conclude the game installation. Just use the touchscreen.
Blurry text
To avoid artifacts on screen, make sure to untick _FXAA in the graphics game options
Unbearable permanent purple screen flickering.

Works flawlessly without any tweaking. Online Co-Op Campaign is a go!
This isn't a Proton problem, but worth mentioning that the Match Making servers are no longer up for this game, but you can still invite friends and play. Go hunt some bugs!
Now ACM deserves platinum
It looks like Valve has enabled CEG support in linux. Even 8 months ago I had to look for a free ACM.exe . Now everything works out of the box.
Runs out of the Box. No extra Settings needed. Proton Exp.
Some stuttering but not that bad.

Colonial Marines should be moved Platinum now!
It lags a little just in some points of the single player campaign but is really nothing you can't manage.
If you play Vanilla (no mods) everything works out of the box with 6.3.8 module. The AI, mod or not, is the awful mark but co-op and Bug Hunt online saved the day. A particular honorable mention goes to the audio with the Sound Blaster Z through Pulse Audio system: stunning!

Aliens CM should be moved officially Platinum now
If you play Vanilla (no mods) everything works out of the box with 6.3.8 module. The AI, mod or not, is the awful mark but co-op and bud hunt online saved the day.
Since CEG is now working, this game works flawless
Tabbing out and back into the game, might show a small Window, doing it again fixes it.

Comparto el enlace del ACM.exe que funciona solo con "Proton 5.0-10 o inferior": https://drive.google.com/file/d/10D0Y6IchqXaQaYp16aOZM4RVWtFiTBPy/view?usp=sharing descargar el fichero "ACM.exe" y reemplazarlo en ".steam/steamapps/common/Aliens Colonial Marines/Binaries/Win32/ACM.exe"

Este juego funciona muy bien con "Proton 5.0-10" o inferior, se demora 1 minuto en abrir.
Sólo hay que esperar. no es necesario "alt-tab" ni nada por el estilo.
As of 29/10/2021 CEG DRM is working with Proton experimental on the Beta client of Steam.

Game launches and is playable out of the box using current proton experimental
Game launches minimized/in the background, have to bring it up manually by alt+tab
Works grate after replacing ACM.exe as descried by Illusion
You have to download: Aliens: Colonial Marines v1.0.210 [M7] Fixed Files (https://gamecopyworld.com/games/pc_aliens_colonial_marines.shtml)
Extract and only copies the exe ACM.EXE to steamapps/common/Aliens Colonial Marines/Binaries/Win32/
Steam DRM
Grab it from here: https://gamecopyworld.com/games/pc_aliens_colonial_marines.shtml
Look for: Aliens: Colonial Marines v1.0.210 [M7] Fixed Files
Extract everything but only copies the ACM.EXE to steamapps/common/Aliens Colonial Marines/Binaries/Win32/

Doesn't start.
Completes first-time setup of UE3 runtime, but the actual game does not progress. This is because the game is utilizing an older version of Steam's DRM which is, somewhat ironically, not supported by Proton. Tried using a DRM-free executable as previous poster suggested, but no joy. Interestingly, if you install Steam through Wine and run A:CM that way, it runs (albeit at 15fps).
Needs to replace the ACM.exe with a free-drm ACM.exe to start and play the game.
NOTE: CEG Game(Steam DRM)
The game does not start.
Won't start at all

Tried reverting to 3.16, but apparently steamVR/openVR brakes the dot.net installation.
It just indefinetly hangs trying to access "~/.config/openvr/openvrpaths.vrpath", despite me not having any VR devices, or steamVR installed for that matter.
Used to run fine. Downgrading to Proton 3.16 doesent do anything, so i guess one of the recent MESA updates broke it.
No chance to get this one running. It installs just fine but won't run. Steam tells you it runs but after a few seconds stops. RIP

The game is completely broken
I was able to install the game and go through pregame installers (C++, .Net Framework, etc). But after that, I wasn't able to run the game itself. None of the Proton launch options didn't help.

When launching the game, the .NET installation appears to go through now. But when launching the game nothing happens. Steam just says game is running for a while.