Sonic Unleashed has aged like wine; maybe it's not the greatest wine of its day, but time has sweetened it significantly. It's still visually stunning even if it only runs at 880 × 720, the soundtrack is chock full of bangers, the gameplay is solid, the handful of glitches and exploits that are there are quite fun, and the story is pretty enjoyable even if it's nothing special.
The other part of this game aging well is that nearly every mainline Sonic game that's followed Unleashed has not been up to par, with the major exception of Sonic Generations (Sonic Mania can also apply here, but I like that game for very different reasons than Unleashed and Gens). I don't really like Colors and was left deeply disappointed by Lost World and Forces, and it's really made me appreciate how much work they put in to make this game great. It also made me wonder why Sonic Team seems unable to push for quality beyond this now.
(Oh, also, the game got a 60 FPS patch on Xbox Series X|S and runs much better on those systems even without the patch. That's also kind of important to the game aging well, but I couldn't think of a less intrusive way to include this point.)
My main issue with the game today is that I don't really think any of the night levels are worth returning to. It's not that they're wholly devoid of fun, but rather that they don't have a valuable intrinsic reward for playing the stage again, making a large chunk of the game feel somewhat disposable after you max out everything. It's a shame too, since this is the most that a Sonic game has fleshed out a combat system, and outside of replaying Eggmanland, I almost never use it.