This game deserves much more than a 4.8 but still it has its problems.
There is one mistake sega made in this game and that is GUNS and VEHICLES. The gameplay is somewhat life sonic adventure 1 and 2 but but with MAJOR PROBLEMS. Sure you do run fast in the game but there are many times when you have to slow down and it makes the game feel awkward. Also a lot of the enemies now have health bars due to the fact you get guns and you have to use the homing attack on them several times. The guns were implemented so badly they seem like they were just thrown in there. They don't have a crossahir, and you can't aim it up or down. So basically your just shooting in front of you. Because of this it is hard to shoot from afar and this makes guns pointless aside from the few more powerful ones. The missile/rocket launcher is actually a good weapon because it seeks. Another interesting weapons is the gun that sucks up enemies. They also have swords and polls you can pick up but there just as bad if not worse. The vehicles are also a major disappointment. The jeeps and cars throughout the levels go slower than you do on foot, and have terrible turning. Luckily you don't have to use them. The walker vehicles are similar to sonic adventure 2 battle but clunkier and most of the time you have to use it for part of the levels. Luckily not many of them are thrown in. The flying creatures are probably the best of the lot, but they still don't feel polished and are somewhat difficult to handle.
The interesting part of the game is you can finish a mission 3 ways. Light dark or neutral. Each mission you finish in a certain way takes you on a path. Some of the cut scenes will be different and the missions are different as well. However some of the objectives tell you to kill all the enemies or allies on the level and this gets REAL TEDIOUS. This takes out all the sense of speed. Luckily most of the neutral mission have you run to the goal
The last thing that sucks is the voice acting. It is terrible. Tails sounds like he has sucked on helium, rouge sounds really old and charmy sounds like he is high on drugs.
Overall shadow the hedgehog is a pretty mediocre game at best and while some parts can be fun and entertaining other aspects are broken. Only diehard fans will enjoy this. To all others, pick up sonic adventure 1 (dx) or sonic adventure 2 (battle).