Play it if: you want to play a medium-length game emphasizing clean, forgiving gameplay and a detailed, engaging setting.
Don't play it if: you want plot-heavy IF that hits the narrative highs of later masterpieces such as Anchorhead.
Christminster to me suffers from the rather thankless role of being a classic game overtaken by its successors. It's a shame because in many ways Christminster is contemporary in its design: lower on cruelty and higher on fairness than most of the Infocom classics. In that respect it's the sort of game that will outlive the Zork series, whose entries will more often than not frustrate the contemporary player in spite of their positive attributes.
I think it comes down to the balance between puzzle-solving and storytelling. For every area in which Christminster presented clean, quality gameplay, there's another area in which it falls just a little bit short in its narrative. Yes, the puzzles have both variety and verisimilitude, depending as much on the manipulation of characters as on that of everyday objects. Yes, they're (mostly) well-clued and engaging enough to keep you playing through to the finish. But then I have to stop and wonder why I'm researching the alchemical history of the university when I should be demanding a police investigation into my brother's disappearance. (Spoiler - click to show)And concocting the Elixir of Life is all very well and good for a puzzle, but what is going through Christabel's mind when she is making it? What is the connection between making the Elixir and saving her brother?(Spoiler - click to show) These sorts of details by no means ruined the game, but they did prevent me from really connecting with Christabel and by extension the actual plot.
The thing is that Christminster feels like a prototypical version of Anchorhead. I know it's not really fair to judge one game by the standards of another, but Anchorhead, which might be considered this game's spiritual successor, really did do it better. The personal stakes are higher, the environment is more atmospheric, the backstory and research more detailed and engaging. Christminster paddles along at a good pace in terms of gameplay, but the plot itself changes very little between the beginning and end; nothing of real emotional significance happens until the ending, and there is little build-up to the climax. It works more as a string of well-connected puzzles than as an actual story, whereas Anchorhead managed to balance both of those elements.
So is it a masterpiece? For its time, yes; and even now it has aged extremely well - the original release was in 1995, but it may as well have been yesterday (barring a couple tell-tale gaps in implementation). So it's still entirely worthwhile as a game, if not as engrossing a story as it could have been.