Myself, I dislike Unleashed, but still find plenty to enjoy in it. I guess I should talk about the HD version, since the SD version isn't even worth the time.
For one thing, I enjoyed the game's tone. The story itself is kinda barren and leaves much to be desired, not to mention that it has Chip in it, the Jar-Jar of the franchise. But the game knew when and when not to take itself seriously, and pulled off both tones really well. It's the style I think all Sonic games should follow, rather than the overly-silly nature of the most recent few games. And it was funny! Remember when Eggman got pegged in the face with a rock?
The music was also great: diverse in instrumentation and tone, depending on the level. The songs were tailored to each fit the culture of the level you were in, but still sounded like something you would hear in a Sonic game. I prefer Jun Senoue's stuff, but it was still an excellent soundtrack. Rooftop Run is one of the best level songs in any Sonic game ever.
The world itself was also diverse and gave the game a sense of scale I haven't felt in a Sonic game since Sonic Adventure. The unique cultures of each country and vibrant atmospheres made the game a joy to just take in, mainly in part to the absolutely amazing graphics. The hubs had interesting people and the entrance stages were chocked full of secrets and places to explore, and getting to the level was half the fun.. unless of course you didn't have enough sun and moon medals, but we'll get to that shortly. The only problem was traversing the "entrance level" portion of the hubs, due to how slippery Sonic controlled at low speeds. This was fixed in Generations though, so if this concept returned I feel it would work beautifully. Oh, and the art-style was also spot-on for the franchise, being super cartoony and stylized as opposed to super-realistic like Sonic 2006 tried and failed to be. In fact, it worked so well that it's the style that the franchise has stuck to since then.
The day-time stages were, overall, really fun, and set the blueprint for the franchise's future by merging 2D and 3D gameplay. Platforming was indeed present, but took the backseat to blazing through the level as fast as you can. Your score was dependent on how cleanly you made it through the level, by relying on making split-second manuevers in order to keep the pace going. These kinda levels made you feel like you were Sonic, like you could finally do all the cool shit you saw Sonic do in the cutscenes of the previous few games. The only real problem I had with these levels, besides the slick controls at slower speeds, was the boost. I still have mixed feelings about it, as it totally removes the need to include any momentum-based physics, nullifies the usage of the spin dash, and makes enemies a complete joke as Sonic can now just run through everything without fear of any reprocussions. I even had this gripe with Generations and hope they can find a way to limit the boost and reintroduce the feautres that the franchise was built upon. Still, for what it was, the day stages were solid, and definitely better than anything that we've gotten on a home console since the Adventure games.
The problem comes with the night-stages. The reason that the day-stages barely focus on platforming at all is because Sonic Team decided to contrast the day and night stages as much as possible, making the day stages feel like racing levels, with the night stages containing almost all of the platforming. But they also decided to combine the platforming with combat. And, for the most part, the combat is actually done well. Rather than being a dumb button masher, you're encouraged to use combos to take out as many enemies as possible, as quickly as you can. And hell, there's actually a lot of combos you can use, so what is there, is well done. But given the Werehog's lumbering pace and focus on combat, as well as slow and methodical platforming as opposed to high-speed platforming, it's definitely alien to the traditional elements of the franchise.
And that in and of itself is not bad, but the problem is that the game FORCES you to play as the werehog. What's more, in total, his gameplay accounts for 50% or more of main story. There are times when, after playing a 3-4 minute day stage, you cannot progress the story until you beat a 7-8 minute Werehog level that feels absolutely nothing like the day levels, and it doesn't even matter how many medals you've collected. This jarring change of pace not only contradicts the entire point of the franchise, but even when you play it as a stand-alone game, it STILL feels disjointed and unfun. After getting immense satisfaction by blazing through a level, the game expects you to have fun by slowly meandering through environments where you have to backtrack to find the items you need, so you can slowly progress to the next designated area, in which you cannot progress until you eliminate all enemies, so you can slowly progress again to an area where you have tight-walk carefully over a chasm at the speed of a snail with parkinson's, then rinse and repeat until you want to hang yourself.
The werehog on its own wouldn't be so bad if the levels were designed for him to move quickly through them, but they're not. They're designed exactly in the same way I mentioned, and it's not fun whether you're a Sonic fan or not. It doesn't matter that these levels completely contradict the point of the franchise, they just suck on their own merits. It just happens to add insult to injury when you're doing this stuff in a Sonic game. I mean, hello, the main character's name is SONIC, what kind of levels do you think the fans expect in a SONIC game?!
But beyond the fact that these levels are longer than the day stages, you have to progress to the next levels by collecting sun and moon medals. Moon medals allow you access to the night levels, and sun medals allow access to the day levels. The problem is that, for some ungodly reason, the majority of moon medals are found in the day stages and the sun medals are mainly in the night stages. So that means you have to drag your feet through the night levels and search as thoroughly as fucking possible in order to find the sun medals to get to the next day stage. At times, this can easily double the time of an already bloated night level. If the Wii version of the game did anything right, it's that you were awarded sun medals and moon medals based on how high your ranking was after beating a level, and you actually got sun medals for beating the day stages and moon medals for beating the night ones.
And that's why I dislike the game as a whole. It's the same problem Sonic Adventure 2 had, but amplified, since the Werehog doesn't even move very fast. I feel that if the game focused on tightening up the day-stage formula, the game would have been excellent. And hell, they still could have done something creative with the day and night cycle. For example, the entire game could have been played as normal Sonic, but depending on whether or not it was day time or night time in the level, there would be different challenges, like the Dark Gaia enemies would only come out at night, or you could only access certain paths of the level during the day. That would have been awesome.
But as it stands, we have Unleashed, and I think it did a lot of good for the franchise, both by setting the blueprint for the games to come and by affirming that shoehorning in a completely different gameplay style to flesh out the game was no good. Gimmicks should enhance the overall style of the franchise, not completely can it. Totally different gameplay styles should be saved for the spinoffs or minigames.