This wasn't the first Apogee game I played - that honor goes to Duke Nukem - but I found Keen 1 to be a more challenging, and more rewarding, game than Duke. The realistic motion, with the inclusion of fairly well simulated momentum, added a measure of realism that was missing from many other platformers. I'm still not sure that any other game has done it as well, including Keen 4.
At his desk, my father used to have a picture of me playing this game very intently, and I have to say it's an accurate portrayal of how I remember Summer '92
I'd never played the other two episodes until a just a few years ago. They unfortunately struck me the way most other registered versions of platformers did - just more of the same. Sometimes I wonder if Apogee was shooting themselves in the foot with the shareware model, at least earlier on.