Earth and Sky is a popular piece that is probably most notable for two things: introducing the superhero genre to IF, and successfully founding the first serial adventure in the medium. [edit: After writing this, I realized that the Frenetic Five series probably has a better claim to both of these distinctions.]
An IF Comp entry, Earth and Sky takes no chances with the recommended timeline for gameplay; it seems impossible that you might not finish within the allotted two hours. Its relatively linear story -- usually a big turnoff for me -- is compensated for by the fact that this story is fun to read and still somehow offers a sense of freedom. Rather than feeling railroaded by the plot requirements, you feel swept along by the fast-paced events you are involved in; instead of feeling pushed, you are racing to keep up.
Like the early Lone Ranger or Flash Gordon serials, this work is unapologetic in its use of cliche conventions and its cliffhanger ending. It has no need for apology -- indeed, it shows us why those cliches exist in the first place, and the authors have delivered on their promise for more in the game's two award-winning sequels.
Though I haven't felt the urge to replay the game since I first tried it a few years back, this is a fun one that's well-suited to a lunch-hour diversion, or as a friendly introduction to the form for an IF newcomer.