Just some comments related to the guys (e.g. pirate593) who complain about the difficulty of the game. This comment contains some useful hints, so those who want to find out their own strategies and explore the game by themselves, please do not read further.
A note: the 'grabbing' means an alien grabs your marine by appearing during the aliens turn and you can do nothing with it. Most of all other aliens could be killed, so if you are grabbed then you have really bad luck or you play to risky.
The game is playable, however, you must have very good strategy (how to move, when to shoot, choose appropriate order of the marines, ...). Yes, you can have very bad luck in case aliens grabs a marine, however, if the other marines are nearby, it is a good chance to save the grabbed one - so it is very important to move the marines in a good formation so they can help each other every time. Follow this strategy, you are almost able to eliminate 'grabbing' (yes, a marine could be killed immediately, then it is a bad luck). This is useful advice for all levels.
Generally, I have found it is better to be slow and safe - it means it is better to shoot than do risky moves. This is good strategy especially for the Reactor level.
The Operations are very good playable even if you have only the default characters (Gorman, Hicks, Vasquez, Hudson and Ripley - unfortunately, to have less than three fully armed - not those with pistols - marines make the level almost unplayable). The hint lies in putting the marines into right places and use grenades. Then you can save all of the marines. In the tunnel, aliens appear always in the same key. Apone or Hicks are good guards in the tunnel (they can do one move and then shoot with a pretty good chance of hit). In case you have almost all marines saved in the Reactor then the Operations level is quite easy and routine work. Yes, you can still have very very bad luck, but in this case you can loose no more than one marine (in really really bad case two of them).
The last level is probably the most difficult. The easiest is probably only in case you have just Ripley, Apone and Hicks. Not so many aliens appear and you have always three moves. But you can save only three humans and trying to kill the alien queen is a bit suicide (you do not have enough firepower). Having more marines leads you to play very gently in a tactic and strategic way - it is a bit like a chess. The good news is you can almost absolutely avoid the grabbing: do not stay near the shafts and after entering the room it is better to go out and wait three turns. The last hint is annoying and time consuming but necessary. Time is your worst enemy. The good strategy: take the Ripley, Apone and Hicks and go for the Newt. The rest of the marines can clean up the closest rooms and prepare the way back.
It is possible to reach quite a good score (IMHO over 7000 points) and save Newt, Burke and the marines. To be able to get such high score you have to train and learn (or be very very lucky). Have a good luck!
Tip: if Vasquez shoots first, you will get 'Let's rock!' 100 points extra (in the Reactor and the saving Newt mission).
P.S. To homedrone: thanks for the nice game.