I beat it last year (around this time) after 4 months of on and off play. It was one of the most satisfying gaming experiences of my life.
Without spoiling too much for those who haven't played: the first playthrough of Fez is basically a casual and enjoyable platformer game with some interesting mechanics. However, the real meat of the game is in the second playthrough. Once you beat the game, you have to re-explore everything in order to complete all of the puzzles that were left unfinished. This is where you start to realize how incredibly deep this game is. You start to notice things you didn't see before. Symbols and images you overlooked or disregarded take on new meaning. Areas you thought you had already completed become relevant again.
Fez blew my mind with its depth. The first playthrough is literally just the tip of the iceberg. There are some ridiculously complex puzzles to solve. I refused to look anything up while I played through Fez, which is why it took me several months to complete. I kept notes on a notepad that came with my copy of Diablo III. I eventually filled the notepad with symbols, letters, notes, and reminders that somehow all came together to help me finish the game. It was so refreshing to play a game honestly without relying on external sources.
Fez brought me back to the feeling I got when playing games as a kid. It was incredibly nostalgic even though I had never played a game like it before. The nostalgia was more of a feeling of exploration and innocence that is sorely lacking in games lately. While games like Battlefield and Call of Duty are pushing graphics, violence, and realism to their limits, Fez is an incredibly rare chance to remember what it felt like to play games as kid. It's incredibly well-designed, beautiful, and genius.
Fez is a masterpiece and is required playing for any adult gamer who isn't afraid to think and be taken to places that you never imagined a game could take you.