The forefather of modern adventure games, Zork brought the genre to the masses. I have begun this game many times, wandered the dark and dusty halls of the Great Underground Empire, only to get completely stuck and put it back on the shelf. Finally, after my wife helped me finish playing through Trinity, we tackled this adventure again and completed it!
There are certainly puzzles here that haven't aged well (e.g. (Spoiler - click to show)figuring out you need to give the egg to the thief or (Spoiler - click to show)defeating the cyclops) but for the most part after a little experimentation and discovering the types of interactions the game expects you to take, most of it is well structured.
There is effectively no story to speak of, and the few other living creatures you encounter are hostile obstacles to bypass, but the immersion here is top notch. The descriptions are not necessarily grandiose and examining the vast majority of objects only states that they are nothing special, but the writing is just detailed enough to let your mind fill in the blanks mostly without even noticing. You feel like you're delving deeper and deeper into damp, rocky caves and where evidence of civilization lies, it's rough, dusty, and abandoned. Unlike the sequel, there is not a huge variety of locations, but each room is exactly distinct enough that you'll start to remember where it is on your map very quickly (you **are** drawing a map, right?!).
Zork isn't a very cruel game, but it will seem like it is until you wrap your head around how the world works (the occasional poorly designed puzzle excepted). It definitely doesn't hold your hand. While I'm usually a fan of starting at the beginning, Zork is difficult enough and the learning curve steep enough that I'm not sure I would recommend it as an entry point into the genre for beginners unless they are already excited and eager to take on the challenge. That said, not having played very many interactive fiction games, I don't yet have an opinion of a better one. Certainly, if you can look beyond the lack of guidance and get immersed in the world the prose reveals, there are many mysteries and adventures Zork I offers.