The additional challenges were pretty easy, once I chose the right ones, and so Shadow the Hedgehog appeared to me. Having never played Sonic 2006, this is the first time I've been exposed to his lack of character. Luckily I quickly defeated him - it would have been nice to understand why I had to, admittedly - and I had my final chaos emerald.
To the final boss, then, or rather the construction yard surrounding it. I had to jump onto broken platforms and slot in chaos emeralds to fix the gears and start the gate up again. I was rather worried by this, since holding on to the chaos emeralds is generally a good thing,
I needn't have worried. Somehow, although the emeralds are clearly visible embedded in the cogs, Sonic(s) still had them when facing down the final boss. And what an awful final boss it was. As Super Sonic, you have to boost forwards towards the big Time Eater enemy, and lock on to his core. But quickly, since your rings supply is constantly counting down and trying to get hold of more rings is pretty difficult. It's easiest to switch to a 2D display until you get close to the boss, then switch to 3D and boost into the core. But it's not easy at all, and took me multiple attempts.
But finish it I did, completing the game.
There's a lot left to do - many challenges in terms of time attacks, races, ring collections and similar. I have about 50% of the red star rings. But I'm not sure I will play much more for now - I'd prefer to play Sonic Colours or a completely different game rather than going over the same stages again. Maybe in the future.
In any case, this is the best Sonic game for a long time, even if the last boss is pretty rubbish.