Game|Life's Susan Arendt has written an interesting opinion piece that basically asks Sega to just stop making Sonic games. It's not that she doesn't like the character -- she does like the character, and has fond memories of when he appeared in good games.As she puts it, "The Sonic games veered further and further away from their simple, yet elegantly satisfying roots, and entered into a territory of uninspired gameplay, pointless mechanics, and craptastic supporting characters." With each game, a few new characters have been thrown on the already-towering pile of anthropomorphic animals who just don't matter at all.
Arendt says that the spinoff RPG Sonic Chronicles does show some promise, but that as far as the main series goes, it's over. The best recent Sonic game has been Dimps' Sonic Rush, which was ... okay. At this point Sonic Team has shown that they have forgotten how to make a decent Sonic game, and so they should just not make any more and let us preserve our memories of when we enjoyed the series.

