In my opinion 'Gunship 2000' is one of the best heli-simulations. Replayability is high due to the many machines you can fly and the randomly generated maps, so no two missions will be the same.
You start by naming your unit and choosing a scenario. Each scenario has its unique characteristics. In the Middle East you fly through large canyons, in Western Europe some missions take place in cities (very exciting as an enemy could wait for you just around the next corner!) and on the Philippines you have to fly around vast jungles (don't try to fly over them or you will be quickly detected and shot down). The toughest scenario is the Antarctica: Not only are here the most dangerous enemies but you also encounter magnetic anomalies which make your compass go crazy, heavy blizzards which greatly reduce visibility and drift off your helicopter quickly, and finally you can crash into almost invisible snow hills.
After choosing a scenario it's recommended that you first fly a training mission. There you are invulnerable and have enough time to get used to the controls. But then it starts to get serious. In single missions you attack radar installments, tanks, anti-aircraft units and many more. There are a lot of different mission objectives: Scouting locations, destroying certain units or buildings and transporting cargo or troops. Sometimes you even can call in artillery or aerial support. Some targets are mobile so you have to look for them in their supposed heading.
Before each mission you attend the briefing where you get your primary and secondary objectives, the latest weather reports and valuable information about what types of enemies you can expect. According to these reports you choose your helicopter and armament. There are agile scouts (Kiowa, Defender), heavy attack helicopters (Apache, Cobra) and a huge transporter (Blackhawk).
Having reached certain ranks you get access to the improved 'Longbow Apache' (equipped with a 'Mast Sight', kind of a periscope) and to two stealth helis, 'Comanche Scout' and 'Comanche Gunship'. You can arm them with gun pods, unguided missiles and laser- or microwave-guided Hellfires (my favourites because they are fire-and-forget-missiles). For very big targets like bridges or ships you can load Mavericks or Penguins. The next step is selecting realism and difficulty levels. You can choose if you want to take care of weapons and countermeasures yourself, if the weather should have influence (wind and visibility) or if you want to fly with a realistic flight model. Four levels of enemy AI can be selected as well.
Then it's time for Take Off! Graphics are ok, mountains are represented by brown pyramids or lengthy formations. The most obvious difference between the scenarios - besides their forementioned characteristics - is ground color. Sounds are ok, too. The most important sound is your co-pilot's voice as he keeps you up-to-date about targets and incoming threats. During the mission you constantly have to be on the lookout as enemy units are lurking all over the place. Therefore a careful planning of waypoints is essential. But it's most fun when you eligible for commanding a flight of five helicopters. You can select and arm each one individually and send them to different routes. That's most helpful in Search&Destroy- or Intercept missions where the targets are sometimes really hard to find. They consist of infantry, tanks, AA-units, aircraft, oil rigs, trains, ships and so on and their types vary according to the scenario. With your flight you can also play campaigns and (of course!) it's up to you to win the war.
After a (more or less successful) mission your results are evaluated and with sufficient points you get medals and promotions for yourself and your crew. If a pilot has been shot down you will be informed wether he was rescued, is Missing or even Killed in Action. An interesting aspect of 'Gunship 2000' is, that you can rescue your comrades yourself in case you have a 'Blackhawk' with you. Just send it to the crash site and it will pick up the unlucky fellow.
So you fly mission after mission until you finally reach the rank of Brigadier General which ends your active career. In case you suffer a heroic death you can always restore the previous roster.
All in all 'Gunship 2000' is a very well made game and you will certainly play it again and again as the randomly generated missions provide new situations all the time. Enemy AI can be a real challenge as well. In Elite-level you have about 2 seconds to react before the enemy unit opens fire on you! Careful route planning and a cautious approach are the keys to success. I can hardly find negative aspects besides the fact that your comrades are quite vulnerable to crack or elite enemies and too often end up on the ground waving for help.
So 'Gunship 2000' is IMHO a true simulation classic and should have its place in every gamer's collection.