This "Complete Pack" contains the first five full Keen titles, excluding Keen Dreams and Aliens Ate My Baby Sitter! probably due to publishing rights issues.
Commander Keen in Marooned on Mars:
The first entry into the series, and a very enjoyable one. The difficulty isn't too high or too low, just about right; it's short and sweet. Even though there is a tad bit of input lag regarding controls, it never really makes you perform pixel-perfect jumps or anything that difficult. And despite the lack of music and somewhat annoying sound FX, the title still retains a lot of its original charm.
Commander Keen in The Earth Explodes:
Out of the original trilogy, this is my least favorite one. Why? Because it's basically the first one, but quickly loses its charm when the unfair gameplay kicks in. And I don't mean "unfair" as in "challenging", I mean unnecessarily aggravating. This is due to the tad bit of input lag, bull**** level design, and annoying enemy AI. Thanks for that John Carmack.
Commander Keen in Keen Must Die!:
This entry is even more unforgiving than the second, and the lack of music and sound FX REALLY wears on you by now. Why isn't it my least favorite? The main antagonist. Without spoiling anything, the main antagonist and his simple (yet relatable) backstory make the game worth seeing through to the end, and you barely even learn anything about him until the boss fight, which luckily, isn't too hard. I'll just say... it shows a lot of humility for id Software, considering how they went on to turn the entire industry upside down multiple times.
Commander Keen in Secret of the Oracle:
This is probably my favorite Keen title, why? Well, one reason is Dopefish's glorious debut. Another is how nearly every aspect of Keen was improved vastly. This is when the Keen series felt much more like a fully-formed franchise rather than a labor of love from a group of eager programmers. The graphics are much more colorful and cartoony, the gameplay is MUCH more fluid and MUCH less aggravating, and there is actual background music that’s catchy and makes the experience more enjoyable. And you can't forget Dopefish, we will never forget Dopefish.
Commander Keen in The Armageddon Machine:
A lot like Secret of the Oracle, except new levels, music, easter eggs, and sadly leaves on a big cliffhanger, and I only mention this because the sixth title is not included leaving you to find it yourself. Still a lot of fun and pretty enjoyable. The only nitpick I really have is how you actually beat the game, as it is very confusing and very hard to actually figure out how to blow the last fuse. Otherwise, a solid Keen title.
Some people complain on how "You're paying for an emulator!" or "It's missing two episodes!” I really don't. This is the bulk of a classic series, and you're paying how much? Five dollars? And you'll probably wait for a sale anyway, won't you. You should be grateful it's even on Steam for legitimate purchase. Also, the games run almost flawlessly in DOSBox, unlike many other DOS games released on Steam packaged with DOSBox...
If you want to explore id's past, want to dive into another retro platformer, or simply just bored, buy the pack now for five bucks. Don't even wait for a sale, it's not really worth waiting for when you could be playing now.