The plot is pretty much just Shadow trying to find all of the answers to his past and it's up to you on whether he goes on the good path, evil path, or neutral. Each has their own mission such but the normal path is just race to the end (like most Sonic games).
The guns are kind of worthless and the vehicles are even worse. However, it's not impossible to use the guns (just face your enemy, jump, and fire) and the other attacks work ok.
Multiplayer isn't that bad after a little while but you seem very limited on what you can do. I do like the concept behind it though, Shadow androids fighting for the title of "the real Shadow".
So, although this game did seem a bit rushed, I don't think that it deserves such a low score. It doesn't deserve any less than a 7 or an 8.