I played through Firewatch over three sessions, it took me almost double amount of the time that @Draikin spent (Steam says 5.8 hours). Typically I take a bit longer than the average to complete games as I like to take my time and observe my surroundings. The average play time is 3.5 hours.
However, the short length didn't bother me at all. Indeed, I was dismayed when I saw a number of negative reviews on Steam that judged Firewatch solely on it's length. It's not significantly more expensive than a movie ticket, but if you really must judge a game on such objective criteria (such as hours spent = value), then by all means wait for a sale.
Firewatch is a game with very little filler. There are some sections which involve backtracking, but these are short enough and add further immersion (and it is a very immersive game), so I never found them tedious. The game knows when to cut to a new scene and when to take you hike to new location. The hiking sections give you time to reflect on the story and way you're role played the multi-choice radio conversations. These sections give you time to think about where the story is currently at and what Henry, the protagonist, might be feeling about the current situation.
Another common objection to the game is the lack of things to do. Depsite the appearance of an open world, Firewatch is a very linear game, and there's not a lot in the world... a large part of the attraction is just "being" in the very dry but beautiful environment and navigating around the world using the paper map (which the protagonist holds with his hand in game, rather than bring up a seperate screen - very cool). I can tell that developers have been very careful about not putting anything in the game that would distract from the immersion, so nearly all of the usual, unessential video game elements such as collectables and stats have been stripped out. Part of the appeal isn't what the game does, but what it doesn't do. Firewatch is not a game that tests you (with either puzzles or combat), the main "game" mechnanic is the radio and how you build your personal relationship with Delilah. Unlike other games (e.g. Mass Effect), where you get achivements for inactivating with characters in the "right" way, in Firewatch the interactions are something that are a lot more subjective. You cannot win, because the consequences of your radio responses are not goal orientated. They are about building something subjective and personal, not about completing a "quest". Indeed, Firewatch is not a game you set out to win via objective criterita (other than reaching the end of the story), it's very much a subjective experience. I actually see a lot of parallels between this game and Journey. And not just the fact that they're both orange games either The experience is one which is ultimately very much about how you personally respond to it. I haven't played Gone Home yet, but have played other walking simulators such as Dear Esther. After Journey, Firewatch has been the walking simulator that has appealed to me the most and I will likely replay it at some point.
So yes, I quite liked Firewatch and recommend it, keeping in mind that if you require that games are "busy", this game is not for you. You'll probably want to wait until you get some decent hardware before attempting to run it @Shadow, as even on my machine with 16GB of RAM and a 2GB GPU, I had to run the game on low graphics to get it run smoothly. The visuals, dialogue, acting, and music are all top quality. I won't say anything about the story, other than that's appealing largely because I imagine it actually happening, which is something I can't say about a lot of game stories. If you're after something fantastical or epic in scope, Firewatch might not be the game for you, but if you want an experience that's very immersive because it isn't those things, you should definitely experience Firewatch.