Huge Sci-Fi FPS? Check. Runs on a computer powered by treadle? Check. Get it for $5 or less? Check. What's to lose?
You can read about the general feel and narrative of the Ukranian-made sci-fi FPS "Vivisector: Beast Inside" (2005) elsewhere, but a quick rundown goes like this: You are a soldier dumped on an island tasked with subduing a Dr. Moreau-like mad scientist who has been doing experiments on animals, creating "humanimals"--yeah, basically leopards, lions, and cheetahs who run around with SMGs and grenades (and claws when they get up close). The gameplay is pretty darn swift as you traverse both interior and exterior environs; the controls are comfortable; the weapons are serviceable (with a few energy weapons to boot)--nothing extraordinary, but they get the job done and feel generally hefty. Due to the game's age, I'm playing it on a non-sooped-up laptop with my 360 controller (wireless reciever for Windows), and other than there being no aim-assist (the game was made to be played with a mouse after all--which makes targeting a bit tricky with thumbsticks), the game runs like a champ--smooth as glass, no hiccups, all high settings (such as it is).
By 2010 standards, the graphics in "Vivisector" have the familiar Source Engine feeling of being lifted from somewhere between HL1 and HL2. The maps are expansive, and if it weren't for the very linear FPS gameplay (typical for the time), this would almost look like a sandbox game (but it isn't.) Some of the lush (term used loosely) jungle areas are so massive, you actually need to use the map to assure you are pointed in the right direction towards your next waypoint. This open-space feeling is nice, and it makes the game feel "larger" than it actually is. The interior sections add some needed variety; in fact, the environs are surprisingly varied overall, with your typical tunnels, railcars, basements, labs...even traditional battle trenches (a test-area where the humanimals practice for war).
While the narrative is at first generic and impersonal, it becomes interesting, and more personal, rather quickly as the pasts and relationships of the characters are fleshed out. The story--rather than the repetitive gameplay--kept me pushing on towards the end. It's a lengthy game--it is at least a 15 to 20-hour play for any normal human being who has a life outside of gaming.
A few words of caution: Stick with the game if you plan on making the time-investment at all. Here's 2 reasons why:
1) Graphically, as well as gameplay-wise, the game (as some others have mentioned here) does not make a good first impression--but it improves significantly as it progresses. As another reviewer noted, it almost feels as though the developers sort of figured out what was and wasn't working as the game was being created. As a result, the environs get better, the gameplay gets better--generally everything improves over time. (I remember turning the game off one night after reaching the halfway point thinking about how disappointed I felt when I first turned it on, but now I was very into it.)
2) For some reason very early on (probably to provide some variety to the gameplay?), the game falls into that locked-into-an-arena-and-never-let-your-finger-off-the-trigger run-around-in-circles type of FPS that I personally find so dull. For about a half-hour straight, your character is locked into arena after arena and waves of Cheetahs with guns are thrown at you relentlessly--a type of gameplay I never admired much. I was so miffed by it (thinking this is what the entire game was going to be), I almost abandoned the game entirely (something I rarely do--even if a game really sucks). But something in my ear said "Give it a chance to see if the gameplay changes." So I stayed with it, and thankfully the game did return to the typical FPS "make your way through the linear map from point A to point B, shooting small bands of enemies around each corner, using stealth in some instances, and scrounging for health and ammo." Yay!
In sum, get it cheap, run it on your system that is powered by gerbils on wheels, you'll be good to go. A guilty-pleasure timewaster.