Pure fun, in small doses
First of all, it's innovative. Years after 3D platform games proved that 2D platform games are mostly dead, this game, in my opinion, still ranks up there as "an interesting platform game".
The graphics are extremely beautiful in all their low-resolution glory. I really like the look of this game, some areas and backgrounds are absolutely cool. Also, most of the ambient sounds are very nice, good enough to give all sorts of shivers and actually create a believable atmosphere.
It's a fun game too. Sometimes frustrating, but fun. Simple-minded job of blasting uglies has probably never been more fun.
This is probably the first game I've seen that features "live" modding of the game (The Cube Engine being the second, and that was out like 8 years later). The moddability of this game was a very interesting concept - shame that the modder turnout wasn't too huge.
Also, great thanks to Crack dot Com for making the game public domain and all of the future modders - SDL port sure helps me to play the game these days.
Basically, the game has a tendency to get boring easily. It is, if I'm to borrow a metaphor, kind of like a game equivalent of holding breath: if you play too much of it, you pass out. This is raw, merciless slaughter in beautiful pixely futuristic environments. Not that there's anything wrong with that... It's exactly the kind of game that you probably play a few hours every year, but no more.
Also, the modding is kind of tricky. There's little documentation for anything; only the level editor is somewhat well documented. The Lisp code looks puzzling to me without proper tutorials - and I'm not a stranger to Lisp. There are a few rather silly examples out there, hopefully, if anyone is to make a real mod.
The game would be pretty damn fun online but finding people who play it is pretty near impossible since there aren't server browsers.
The Bottom Line
Basically, the game is a beautifully detailed 2D platformer, with independent weapons aiming (move character with keyboard, aim with mouse), very nice graphics, and tons of gritty "dark sci-fi" feel.
There's no plot to speak of - actually, the story was changed between the shareware release and the final release, and barely no one noticed. Your job is to kill strangely Alien-like mutants. Tons of them. They can shoot back. There are also less frequent robotic enemies (huge robots that throw grenades, small flying robots that shoot lasers, tunnel boring machines), and traps and environmental hazards. You have arsenal of zillions of nice weapons at your disposal, everything from lasers to flame throwers and energy beams.
Then, there's also multiplayer mode, ability to write fairly unusual mods in Lisp, a simply Tab-togglable map editing mode, and other fun stuff. This game is pretty much a very geeky concept, and I love it. Maybe its unusualness made it less popular than it deserved to be. I think it's still an extremely innovative and fun game, as long as you don't play it all day long.