Don't let all the hate make you up pass up giving this game a try.
As the name implies, StH focuses on Shadow's enigmatic past and his actions for how he finds out more about whom and what he is.
Due to past events (Sonic Adventure 2), Shadow has lost a majority of his memory except for his name and the image of a murder of a girl named Maria. While pondering his dilemma, Shadow witness the arrival of a destructive alien race called the Black Arms. Shadow's indifference to the human's plight is perturbed when he is confronted by the leader of the Black Arms, Black Doom, who calls out to Shadow by name and orders him to bring the seven chaos emeralds to Black Doom as promised. Shadow realizes that Black Doom is knowledgeable of his past and begins his journey to find the seven chaos emeralds in order to find out the truth to the daunting question: Who am I?
Gameplay/Story Progression:
StH tells Shadow's story through a set of six stages. The layout of the world map resembles a tree diagram starting with the same first level on the left hand side of the map, but depending on Shadow's actions, the path branches out to match the mission he accomplished in the first stage. In each stage, Shadow has a choice between completing a dark mission, a normal mission, or a hero mission. Shadow has a partner for the hero and dark missions of the stage and flies solo for the normal missions. Once Shadow meets up with both partners, he can interchange between the two at any time using the left and right D-Pad buttons. Changing Shadow's partner will swap out the mission counter tab which gives a look into how far Shadow has progressed in the mission currently undertaken. (ie 35/40 G.U.N. soldiers eliminated.) Playing a dark mission usually has Black Doom or Eggman take Shadow's partner role, while playing a Hero mission can give one of the many characters in the Hero line-up (Sonic, Tails, Knuckles, Rouge, Espio, and more.) There's a slight bit of a co-op mode when undertaking a Hero mission as a second player can take control of the Hero partner, but the lack of a split screen when the second player joins combined with the camera sticking solely on Shadow makes it difficult to stick together during Shadow's high speed sections.
More about the missions, Shadow has three separate score counters on the top of the screen as well as two gauges that fill up depending on whether his actions are good or evil. Performing heroic acts such as fighting Black Arms members, extinguishing fires, or healing G.U.N. soldiers will add to Shadow's Hero score and fill up the blue gauge on the top right of the screen. Once full, Shadow will begin to glow blue and have several temporary abilities including invincibility, infinite ammo for his current weapon, and a special Hero ability: Chaos Control. When activated, Chaos Control has Shadow fly at warp speed towards the end of the stage (usually the normal mission's goal ring.) On the other hand, when Shadow performs evil acts such as fighting G.U.N. soldiers, destroying the environment, or healing Black Arms members, his Evil score will increase and the red gauge will begin to fill. Once full, Shadow will glow a searing red and have the same abilities as Hero Shadow save for Chaos Control. Instead, Shadow will have an ability called Chaos Blast, a devastating explosion of energy that will wipe out anything within radius. At the end of each mission, scores are tallied and Shadow is given a rank (A, B, C, D, or E) If Shadow completed a dark mission, his dark score is combined with his score for collecting rings and his score for his time, and any points gained for his hero score are subtracted from the total, and vice versa with his dark score if he completed a hero mission.
Shadow's choice between completing a hero, normal, or a dark mission affects how he progresses through the map. For example, finishing the dark mission on the first level will have shadow progress up diagonally on the world map to the next stage. If he accomplished the normal mission, he would progress straight on the world map, and the hero mission will send him down diagonally, and each one of these outcomes yields a different stage, thus leading the story to have multiple arcs and endings.
Some events are contradictory at times, (i.e. helping eggman with his defenses one mission and fighting him right after.) Also, no matter what side Shadow is on, the enemy AI always looks at him as a threat, thus weakening the experience of fighting for a side of good or evil when a supposed ally purposely attacks you. This standalone experience could've easily been fixed possibly by making either the higher hero/dark score in the stage represent the side Shadow stands with and have the AI distinguish this as whether Shadow is a friend or foe or maybe have the AI recognize Shadow's current partner and behave accordingly. Evil AI doesn't attack when paired with an Evil partner and Hero AI doesn't attack with Hero partners. The "Everyone Attack" method could've aesthetically been implemented as a balance to the normal missions since they're usually the easiest to accomplish and that would logically follow the AI's supposed confusion towards Shadow's allegiance. This flaw in the Hero/Dark karma system makes Shadow's actions seem to only matter on choosing the next level or during cutscenes and have little impact on the gameplay itself and it greatly detracts from the entire premise for the game's purpose of molding Shadow into being a harbinger of destruction or a savior to mankind.
Music/Dialogue
Despite what reviewers say about the Sonic games over the past ten years, the music has always shined. Each stage has its own sound that blends perfectly with the atmosphere of the stage. Whether Shadow is dashing through the floating ruins, combining an archaic resonance with high flying intensity or speeding around the space colony ARK with quiet electronica, the music flows seamlessly. Shadow has 6 themes total, the main theme "I Am" and the other five play during the credits depending which ending Shadow accomplished (Evil, Slightly Evil, Neutral, Slightly Heroic [Chosen One; My favorite], and Heroic.
The voice acting usually sticks to the sub-par level in terms of emotion, but at least every main/sub character has a distinct voice.
Changes to the Formula
Several tweaks to the classic Sonic formula bring some fresh variety to the game style. Enemies have a health bar and size matters. Instead of most enemies in the past dying from one homing attack, jump, or somersault, the better armored enemies are more suited to fall to the new addition to the Sonic formula: Weapons. Shadow has a wide selection of weapons to choose from and can utilize both human and Black Arms weaponry. The simple firearms can semi or full auto, the rockets can be free shoot or lock-on, and melee weapons make their debut (they have an ammo counter though as well. One ammo = One swing) While the human weapons seem to fire the same kind of bullet, the Black Arms' weaponry is much more varied and gives a more personal choice when choosing a weapon that may have the same rate-of-fire. With the threat of more weapons and friendly fire, Shadow's bound to get hit some point, but luckily, the standard result of getting hit in a Sonic game has been drastically improved.
In the past, getting hit in a Sonic game means losing all your rings and only having 20 to pick up afterwards if you had more than that limit. In StH, getting hit means only losing 10 rings from Shadow's total, greatly extending the possibility of living and not requiring a mad dash to collect what you can since Shadow's whole reserve isn't ruined upon taking damage.
Another addition is the vehicles. Some vehicles are just regular cars, but some take the form of robot walkers with turrets. There are also some scenes where Shadow rides on the back of a Black Arms bird creature. One type of vehicle is a hover disk which Shadow rides to cross hazardous terrain, but sometimes these mandatory vehicles slow down the high speed pace.
Lastly, StH introduces some collectibles in each stage in the form of keys. These keys open a secret door in each stage, which can sometimes reveal a shortcut in the stage or other rewards.
Conclusion
There are over 300 possible combinations for how to play through StH, and the multiple story arcs and endings keep things interesting until the true ending (Last Story) after seeing all previous endings. Keep an open mind and you may just be pleased with StH as I was.