I'll start by saying that I loved Stalker: Shadow of Chernobyl and so it was with great anticipation that I looked forward to playing the sequels Clear Sky and Call of Pripyat. Even though the reviews for Clear Sky were not as positive as the other two, when you run across a good game, you're hopeful that its sequels will be not be complete dogs.
Clear Sky is actually a prequel in time to Shadow of Chernobyl. Does it matter which you play first? You don't play the same character, but both games use many of the same locations and will be instantly recognizable. However, if you like the story in order, Clear Sky would be first. However, as consensus agrees that it's the weakest of the three games, it might color your decision on purchasing the other two, which would be a shame. Personally, I found all three games to have their own enjoyable identities.
The story takes place after the disaster of Chernobyl in a location that's known as the Zone. The Zone experiences mysterious emissions and is full of mutants and strange anomalies that can produce valuable artifacts with various properties. Many factions are fighting over this area either for profit or for scientific research purposes. You play a Russian mercenary who has inexplicably survived one of these deadly emissions, but receives the bad news that the effects of this will eventually kill you. Exposure to one more of these emissions will result in your death. Researchers believe that these emissions are being triggered by a group of stalkers who have found a way through the anomalies and radioactive areas into Chernobyl itself. With your life at stake, you embark on various missions to find and stop this specific group.
Stalker is what is referred to as a sandbox game. Although there is a linear story, there is so much else to do that you can make these games as short or as long as you want. Some might be put off by the learning curve as in the beginning you are often not sure what you should be doing. Although Clear Sky is not the best of the series, it could be argued that playing it first is good training ground for learning the practical side of the way these games are played. You will be well learned by the time you play Shadow of Chernobyl.
The first two levels, Swamps and Cordon, are basically a snooze, and are mostly about collecting items, but things heat up when you get to Garbage. By this time, you should realize that many of the tasks are repeating and can be ignored if you want. These just provide ways of increasing your loot and equipment. However, missions can also make you friends of various factions and can effect certain events that will make your game unique. Also, some tasks need to be done or you cannot advance. You also need to talk to people to get further missions.
There are several weaknesses in Clear Sky. You're practically tripping over equipment and supplies and you could spend hours collecting and trading these things if you're obsessive about it. Weapons are literally scattered at people's feet and you can pick them up and immediately trade with that person. This is not very realistic, or challenging. Also, although the large PDA is very useful, when closed, the small representation of it isn't nearly as useful and you find yourself having to open the map during awkward moments, like during a firefight. I also found it very frustrating that many of the names of the locations were not posted on the map making identification difficult until becoming very familiar with that area.
As to combat, when you are on a mission with the faction, you often only have to kill one or two enemies to complete the task and get the reward. Later though, you will be up against many on your own, which is the better and more challenging gameplay. Night also seems to be much longer than day, but you can use your sleeping bag in a safe location to advance the time. Also, when you hear a call for help on your PDA, you can start running in that direction only to get a cancellation before you have time to get there. Often this results in a failed task. Eventually, I learned to ignore these if I wasn't very close to the location. Chances are the same call for help will come again as the factions are continually fighting over a position.
The trick is to learn what is useful and what isn't. For instance, regular ammo is practically useless except for the mutants as everyone is wearing armor. You don't need food very much, so don't carry a lot of it, and health, bandages and cash are easy to pick up.
The game controls are easy, but one deficiency has you unable to bind the mouse wheel, thus making changing your weapons a pain. Another complaint is about the inaccuracy of the gun sights, but I think there may be a mod for that. Although there is quicksave and load, load times are atrocious, so try not to get killed. (ha! ha!)
All in all, even with the game's weaknesses, Clear Sky was an overall satisfying experience. I've touched on the negative aspects, but it has many positive features. I heard a fellow say he wasn't paying attention to the story, but it's the story that makes these games so interesting. If you don't like stories and only want combat, you're playing the wrong game. I'm also in awe of games that span vast areas and the graphical environments remain so incredibly detailed. I don't hesitate to recommend these games with the caveat of researching potential issues first, as these games will need fixes.UPDATE April 2015: Gog is offering the ability for owners of a Retail boxed version of Stalkers games to redeem them free on Gog with the original CD key. Gog Reclaim