Finished Sonic Adventure 2 in about 6 hours over four (and a bit) play sessions. I intended to do the last few stages of Dark and Last Story in the same session but I started a little late and had to do Last Story today instead.
In terms of basic mechanics, the game's fine. It (mostly) works, though it's pretty painful when it doesn't, and there's a few things like the Homing Attack having no indicator for when something's in range or what it's aiming at that were done a lot better in later titles. Not having to charge the Light Dash helps avoid breaking flow in Sonic and Shadow's stages, but it can be a little finicky to get the timing right at speed. Not having the hub worlds in between stages helps the pacing compared to Adventure 1, but having to track down upgrades in the middle of stages kills the pacing whenever they're mandatory to progress and you have to take a detour to find them, which is more often than not. Knuckles's stages are a pain, and while most of Rouge's are better, Mad Space is just not fun. Tails and Eggman ironically play the best due to not being too fast for the camera to keep up with. Ultimately, though, where the game's really let down is in its level design; aside from a few standouts, it's just not very good, leading to a lot of stages and bosses - particularly, several points where there's several in a row that aren't fun - that make the game kind of a chore to play.
The story does get better in Dark Story, partly because compared to Team Sonic (summed up by Sonic himself, "What you see is what you get!"), Shadow, Rouge and Eggman are more rounded and complex characters suited to a darker and more thematic story. And while being foils to Sonic, Tails and Knuckles wasn't a bad idea, the follow-through isn't totally there. Last Story does manage to stick the landing, though, all plotholes aside.
The story is the best part of the game, but is it worth playing through all 31 levels and about a dozen bosses to get to? YMMV on that, but in my opinion, I don't think I'll be coming back to this one for a while. And I'm definitely not going to try and get all 180 emblems; that's for people with a lot more free time and patience than me.
What's next: Probably Final Fantasy 7 Rebirth once I'm no longer bound at the hip to Old School Runescape.
A new day brings new adventure. But for now... Rest easy heroes. [sic]