Okay this game is something very difficult for me to explain why this rating

Is a good game? No
Is it a bad game? Mmmm yes but not that much

The problem beyond the fact that you are forced to play this thing 10 times (I know there are different routes but that doesn't help) the levels have a fairly inconsistent design. Most levels are straight lines, pretty simple even for a sonic game. And when this is not the case, they are, humorously, by far the most annoying and tedious levels to complete their missions (Mad Matrix candidate for the worst levels of the entire franchise)
Their missions don't make a difference either, most of them are killing x number of enemies, although well, unlike the levels when a mission varies, it is decent like Lethal Highway or The Ark

The game, rather than being unplayable, is... inconsistently flat, it feels empty but when it varies it sucks most of the time. I don't consider it one of the worst Sonic games at all, because even with all its problems one can enjoy it (look at me, I prefer this to Sonic Heroes xd)
I would recommend it out of curiosity only, the game as such is mediocre, bordering on decent