Even giving this game the excuse of being very old, the number of bugs and effort required to play bring it down.
As I recently discovered, the game has an 80 item limit. Meaning that your soldiers, base and aircraft can only have a total of 80 items between them before the game begins deleting items or glitching very badly as alien bodies and it's items during base defense missions also count toward your item limit. In other words, every base you own in the game can only have 80 items total, this includes spare ammunition for weapons and aircraft, tanks and their ammunition, alien artifacts and bodies, etc. You hit and exceed this 80 item limit very easily.
This one factor of the game means that you can't standardize soldier equipment (not everyone can have a grenade, motion sensor, med-kit, etc) or have a large reserve of soldiers and ammunition. You must carefully plan your entire strategy around getting by on the bare necessities and laser weapons become more valuable than the harder hitting plasma, because laser does not use clips that take up valuable space.
This along with a host of other game ending bugs, the ease at which you lose soldiers, the difficulty breaching a UFO since it will be a long time before you have anything that can blow a hole in one (but interceptors shoot them down with conventional explosive missiles somehow), etc all contribute to a game that feels more like work and less like play. Even your victories are short lived, as often times you will be spammed by UFO detection warnings on the geoscape or base defense and terrorize missions.
Even with the support of files such as xcomutil, which repairs some but not all of the bugs (most notably it doesn't fix the 80 item limit bug) the game will be hard to play without frequent restarts due to the 80 item bug, base cave-in bug (where dirt fills in your base and you must dig yourself out using explosives), etc. Furthermore the game is very difficult to get to run in full screen mode, leaving you to play the title in a tiny window which makes browsing YouTube or Facebook very easy while the long missions play out, but does little to help with immersion. The ground missions can last an extremely lengthy amount of time, as you must move very slowly otherwise an alien will shoot you from all the way across the map despite the fact you can't see them. The pathfinding in the game is atrocious, often your soldiers will pick the longest route possible and waste time units or they will move into a dead end trying to reach the target location instead of going around the blockage.
Link to the many, many, many bugs present in this game. The list is pretty big, and many of the bugs will result in having to restart the game entirely. I wish I had found this page of bugs before I spent money on the game or at least before I started playing, I might have avoided some of them. However the need to alter your gameplay style to avoid these bugs is still a major issue with the game.
http://www.ufopaedia.org/index.php?title=Known_Bugs
All in all, you'd be better off saving the $4.99 the game costs on Steam and pre-ordering the new XCOM: Enemy Unknown.