Sonic Unleashed for the Wii trips over its lack of direction, but makes up for it in its overall satisfying gameplay.
It should also be noted that this is addressing the Wii version specifically; the author has not played the other versions, but has been assured that there are significant differences between them. Thus, prospective buyers of the other versions should take caution before applying anything said here to them.
Sonic Unleashed begins with Sonic attacking Eggman's space base as Super Sonic, blazing through its defenses with the usual invincible ease associated with one who has collected all seven Chaos Emeralds. Eggman is prepared for this onslaught, however, and manages to trap Sonic in a field in which he strips the Chaos Emeralds of all their power. As a byproduct of this event, Super Sonic is no more, and Sonic has lycanthropy infused in him, turning him into a slower but more powerful "werehog" whenever nighttime falls. In addition, the world splits apart and Eggman's true plot is laid bare: he wants to harness the power of Dark Gaia for his own purposes of conquering the world. Meanwhile, Sonic is sent careening towards Earth, his Super-Sonic power of flight stripped from his body, and he must find a way to stop Eggman's schemes once more with his newfound mysterious friend Chip.
Over the course of the game, Sonic must travel to many different locations, including Greece, Egypt, and China (although all names are changed to fictional variants such as Apotos, Shamar, and Chun-Nan). The sequence of events is pretty straightforward - Sonic must first talk to people in the town to gain necessary information or items, and can then proceed to the "Gaia Gate", through which Sonic enters into the gameplay section of the game. There are daytime levels, during which Sonic is his usual speedy self, and nighttime levels, during which Sonic turns into his werehog alter ego and has to rough up some dark baddies.
With regards to the story, however, probably one of my biggest complaints about Sonic Unleashed - this is as good a segue as any into that topic - is a problem that has plagued many of the recent Sonic games, including his last Wii endeavor in Sonic and the Secret Rings. That complaint is simple: I can't help but get the sense that the folks at Sega are really not quite sure what they want their Sonic games to be.
On one hand, there is quite a clearly established story that plays out throughout the game. But, on the other hand, the actual events during the gameplay itself really have little to no bearing on the gameplay, being instead bite-sized levels in which the sole goal is to get from point A to point B as best as possible. The game feels quite fragmented as a result; it feels as though over here there is the story, and over there is the gameplay, and they don't really speak to each other. It really just feels restrictive and confining, and I can't help but feel that Sega could have a much better game on their hands if they'd only drop the arcade aspects and make a completely integrated game.
But, the author's personal complaints and suggestions aside, however, the gameplay itself is quite solid. The daytime levels as speedy Sonic in particular can be quite a rush, and are quite a faithful rendition of the quickness and style for which Sonic is famous. The controls are very similar to Sonic and the Secret Rings with the welcome exception that Sonic is now completely controllable with the nunchuck instead of moving on his own. Sonic runs, jumps, dashes, and slides, and that's pretty much the full extent of his repertoire, but not much else is really needed.
The level design is quite well-done in the daytime levels, as well, offering the player many obstacles and alternate time-saving paths that skilled players can take in an effort to improve their completion time and letter grade. Most world locations have one main daytime level during which the only objective is to get to the goal as fast as humanly (well, hedgehogly) possible. Like in Sonic and the Secret Rings, there are also many mission-style levels, in which Sonic must do things such as collection a certain number of rings within a time limit, reach the goal without breaking any objects, or such like. These add a certain level of replay value for arcade-style enthusiasts, but for anyone hoping for a more in-depth story, these will probably detract from the experience overall.
Nighttime levels are, as has been said, not quite as good as the daytime levels, but in the author's opinion they also aren't as bad as some say. They're certainly decent, and the controls work well enough. The only huge complaint that I had about them was the atrocious decision to map the "dash" command to a double-tap of the control stick on the nunchuck. This has the unfortunate effect of making it impossible to edge Sonic nearer to a ledge by simply slightly tapping the control stick again and again. This can very easily cause Sonic to launch into a dash, which is the responsible party in at least 95% of my untimely deaths in the nighttime levels.
But, beyond that, there are no other major complaints in the gameplay itself. Sonic beats up the bad guys pretty darn well, and as he gains power from collecting items and defeating enemies, he powers up both with new moves and with stat increases (the three stats being health, attack power, and his "unleashed" gauge, which allows him to temporarily go into "unleashed" mode whereby he can bring an extra helping of pain to his enemies). They're fairly standard platforming segments, and they work pretty well.
The only more pervasive complaint I would have here, however, is what I said above: the nighttime levels seem kind of aimless, and can be a little boring as a result. One comment I made to myself while playing them was that they seemed kind of like Twilight Princess, only without any real point driving the gameplay: Sonic is ostensibly advancing the story, but in reality his only real goal in the context of the game is to just get to the finish line. This loses a great deal of the appeal to these segments for me; I think that they could have been greatly improved if they had a deeper, stronger connection to the story at large. But, nonetheless, that's water under the bridge, and I won't dwell any further on the subject.
Additionally, when Sonic does well in daytime stages, he acquires moon medals, and similarly he receives sun medals for doing well in the nighttime stages. These can be used to open doors in the Gaia Gates in each world location, which contain short (and surprisingly entertaining) puzzles that can be solved for items, including the very helpful 1-up item, which grants Sonic an extra life at the beginning of each level (he starts with only two, and goes up from there). The author found these to be a neat diversion, and something that he'd have liked to see more of, in fact.
The gameplay and plot aside, other aspects of Sonic Unleashed were quite well done as well. The graphics were top-notch for a Wii game, and indeed the game contained in the author's opinion the best cutscenes in any Wii game to date, although Sega has always been known for excellent cutscenes.
The soundtrack was also well done as well, the nighttime levels having particularly nice music. The one complaint that I might level is the way in which an encounter with enemies in the nighttime levels transitioned off of the level music and into the "battle" music - this was kind of annoying, as the level music always tended to be more enjoyable, and the constant back and forth switching ended up killing any attempt to really get into the music.
All in all, Sonic Unleashed is not the one giant leap for hedgehogkind that Sonic devotees were hoping for, but at the same time it's not the disaster that Sonic detractors were predicting, either. It's a solid game - not the best game of the year - but certainly worth the entrance fee if you're either a Sonic fan or a Wii owner on the hunt for a fun time-killer. Beyond that, however, I can only hold out my (likely futile) hope that Sega will hear what I have to say. I quite think there's a lot of potential to be had, but Sega still seems to cling to the old fractured arcade-style gameplay, which in my humble opinion is one of the things that absolutely must go for Sonic to receive a truly great game.