First, let me begin by saying that this has to be the pinnacle of IF programming. This game is large and deep, and amazingly robust with its responses to player input. I can't even imagine how much time Emily Short put into writing and testing it. Bravo!
As far as the user experience is concerned, this is a great game. It has a well-built environment/world, with backstory for both the setting and the characters. The characters aren't particularly deep, but much more fleshed out than your typical IF game, complete with memories that pop up to reveal more about you (the player character) and the NPCs.
The map is quite large and mostly revealed from the beginning of the game (I highly recommend playing with the built-in map on), but you aren't overwhelmed with possibilities. As you complete the main tasks in an area and clear roadblocks to advance to a new area, you rarely (if ever) have to go back to get an object that you didn't know was important the first time you came across it. I loved that, it both made the puzzles easier to wrap your head around and gave me a real sense of progress as you moved around the map.
The puzzles are revolutionary, using a mechanic that I don't think had been explored before (or since?) this game. It is a nice change of pace from the more mechanical or character-stimulus puzzles of other games. The only downside was that because the puzzles were all word/letter based, it got to be a bit repetitive and a few times a little too easy as it was obvious what you needed to progress and you just had to find an object one letter off from your solution.
I enjoyed this game a lot and appreciate it even more. A must play for any IF enthusiast.