Trine and Trine 2 are great examples of 2.5d games. They are also very beautiful games. I think Trine 2 has the most beautiful artwork ever in a game.
Difference between 2.5 and 3d is that in 2.5 there is a side of a 3d model that is never view able due to control of the camera angles. Since this side is never viewable, it doesn't need to exist, so they can display a 3d model with some of it missing and you never can tell, this is a big optimization.
With the resources saved they can either add more detail to the view able sides, or not and keep the hardware requirements for the end user lower.