There is a lot of game in this game, and if you're into other mythos games, then you already know a lot of how it functions and you're liable to like it.
If you haven't played others, then here's the skinny. You and your fellow players get a character. You have health and sanity. You have several ability scores (not unlike certain RPGs; strength, agility, will, etc.) and you have one special ability. You choose a scenario, and jump in. You move around, discovering the map, rolling dice to overcome challenges, trying to solve puzzles and kill (or avoid) monsters as you figure out what the scenario wants from you. Meanwhile, you're succumbing to the danger and insanity of the nightmarish world around you.
Of three games played, we've won two and lost one, but mostly because we're *very* used to these games and working together on goals. If you're gaming with a new group or not used to how punishing these can be, expect some failures. But that's fine, because of the replayability value.
When you replay a map, you can choose new characters to shift things up, but the app will also offer some variety - different starting items and even changing the actual map itself. I solo'd the 'intro' mission a few times and saw two completely separate layouts. I think the more expansions you own, the more variety you'll see, as it introduces new tiles to the availability.
On the note of expansions - the main game comes with 'only' four scenarios - but one of those scenarios has an average playtime of six hours. The app will save the game for you between sessions, so don't worry about doing it in one sitting, though.
I strongly recommend getting the game organiser that Gameology has, because it makes set-up a breeze when otherwise it might be a little intimidating to begin with. But if you have plenty of little baggies or tubs, that would work too.
As long as you go into it knowing that you can do everything right and still potentially get effed up by random events, die rolls, or just the nature of the cosmos deciding you need to die now, you'll have a great time.

a staple boardgame that most people have played, can't go wrong with it!