First of all, it was (and still is) absolutely free ! Because the game was shareware, this first episode was kindly given to you, in hope that you would buy episode 2 and 3 (you still can!).
Second, if my memory serves me weel, it was one of the first scrolling platform games on the PC.
The gameplay was correct, with several nice features not so commonly seen : the pogo, allowing you to make giant jumps, but which can be controlled in a more difficult way (this is intended, not a control bug !). And also, some ennemies would just push you, sometimes into deadly traps, instead of killing you on contact.
Graphics were more than decent, with funny ennemies and some pieces of nice animation.

The Bad

While most of the graphics were decent, there is no real background to speak of, but this was addressed in the following episodes.
While the gameplay was generally good, it also introduces the "find the yellow key to open the yellow door" trick that would faithfully appear in just about every apogee platform game. While this is not really a commander keen's problem, this is the start of a habit that gets nasty after ten games.
The lastability of the game is abysmal as it can be completed in 2 hours. This, though, is a problem you can easily forgive, as this first episode is kind of a free teaser for the following games in the serie. However, I have no idea if further episodes suffer from the same shortness.

The Bottom Line

Commander Keen is certainly a game to be admired : it was one of the first (and still rare) successful shareware game. It also was a technical achievement, with its scrolling platform action. But, as far as pure gameplay pleasure is concerned, Commander Keen is slightly more than average. A game to be respected for sure. Knowing wether it has to be worshipped is open to debate.