I completed the game, pretty much following my normal schedule for MGS: make it to or just past the boss fight with Sniper Wolf in the snow field on day one (this time it was immediately before that fight) and then finish up the rest of the game on the second day. One of these days, I would like to do a more immersive playthrough of the game, attempting to complete it in a single day, maybe taking a short break around the torture scene, but that is neither here nor there. I tend to play through the Metal Gear series at some point during the summer every couple of years, and I've asked myself why these games in particular are so replayable. More than anything, I think it comes down to the design: If you strip away the story and characters, what you are left with are a series of roughly 8-12 hour games with increasingly refined gameplay and levels precisely designed to accommodate those gameplay differences. This is why the Gamecube exclusive Twin Snakes didn't work as well as the original: they changed the gameplay mechanics to match MGS 2 without altering the design, making much of the game far too easy (first person shooting being the primary culprit). MGS 1 plays primarily on a 2D plane and the design reflects that: guards rarely see you when you are standing on a different level, the Soliton Radar functions in nearly every area, and enemies move in ways that would sometimes make them easy prey if the game had first person shooting. MGS 2 and 3 are similarly designed with specific advances in mind, such as first person shooting and camouflage. In the end, each game feels related to the last, but also unique unto itself, whether it be the outstandingly precise and tight and precise gameplay of MGS 2, the camo from 3, or the mission structure of Peace Walker. The net result is that each game is familiar enough to allow some skills to cross over while still having enough individuality keep you interesting while marathoning the entire series.