Sonic Unleashed basically “appears to be a 3D sidescroller, similar to Sonic Rivals, with occasional shifts to on-rails gameplay with the camera behind Sonic, much like Sonic and the Secret Rings.” Video after the break.
A speed meter can also be seen in the bottom left corner of some screenshots, but whether this has any major gameplay impact has yet to be seen or announced. Also, it seems that there is a ring meter which acts like the “Tension Gauge” in Sonic Rush and Sonic Rush Adventure, which was used for speed boosts
