How Diablo Immortal looks and feels is dependent on which Diablo IV Gold version on iOS or Android, or the open beta on PC. (Your saved data is synced across the two, so you don't need to be committing to either.)
On mobile, the game offers stunning graphics and solid performance. However, you'll require a very powerful smartphone to run it well. In my Pixel 4a phone, the game would lag and crash sometimes, and the phone ran extremely hot. It's also very difficult to keep up with all the action on a screen this small in particular when you have to concentrate on certain abilities. This can be alleviated by playing with a tablet and using a controller -- but that's not enough. You could use the PC.
The PC version is still not at all up to scratch The game is, however, not up to par. It's still mobile games on a large screen, with its poor resolution, and its indistinct textures. Animations for non-player characters are hilarious, particularly the way they are played. The visual issues pale in comparison to the gameplay issues. The game's pathfinding is a mess. rewards do not always load correctly as well; and worst of all, your attack button could occasionally shut down and nothing other than restarting the game is going to resolve it. Imagine how much fun it is when you're at the center of an intense dungeon quest.
On the other hand the sound is good to Diablo's usual standards that include pleasantly atmospheric background music as well as decent voice acting.
I'm not really a fan of Diablo Immortal. However, I imagine some gamers will. It's a minor title within a lengthy series and the F2P buy Diablo 4 Gold mechanics can be as disruptive as they look. If (and this is a big "if") you are able to be able to get past this, there's an excellent game under the surface with a great gameplay experience and lots of other opportunities to explore.