It's definitely aged. Some of the camera work is a bit frustrating. As are some of the controls. And the bits where you just fall through the world for no real reason. Graphically though, I think it holds up and there are some interesting levels like the pinball levels. I also really like a lot of the music.I loved this game. Recently revisited it and if you can put up with some slightly wonky controls, I think it's still great for what it is.
I bought a Japanese Dreamcast on release and I'd only ever really played the Japanese version of Sonic Adventure. It's quite like being able to tell whats going off for once.