Classic Seaside is a perfectly fine Hydrocity throw-back but the modern level feels very thrown together to me, like the 2D and 3D elements of the stage are at odds with one another. Fun, but not something I hope Sonic Team bases further level designs off like Sky Sanctuary.
Speed Highway doesn't have this as much and probably has the best use of the drift move in any of the Unleashed-style games to date. Sort of weird considering it's Iizuka's baby modern City Escape is sorta 'bleh'. Even the remix is kinda half-hearted compared to the classic-mix.
It has its trouble spots, admittedly. The kart section is still a kart section, and the 2D water segment is a 2D water segment. But the overall layout and the way routes flow into one another is excellent and beats the pants off of even Sky Sanctuary, much less everything else in the game.
Speed Highway's not bad. Doesn't branch as much as I'd like in 3D, and similarly there's not much 3D platforming. The Going Down section gets old fast too, but that's an artifact of the original.