The one time I played a DDD in tournament, it was as a CP character on RC when he beat me with his ICs on the starter. I did beat him that match, but it took a great deal of effort because I lost my first and second stocks before he did thanks to the CG-> Dsmash on the boat. Great and lucky position near the blastzone helped me win the match, but DDD is still great on RC and I would think it over before I try to take a DDD (or an IC player due to having a backup) to RC.
Recovering against a DDD, I would think, would be very tough. It's not that Sonic can't make it back onstage, but because every option he can choose when his double jump is gone can be covered by DDD's pivot grab, grab, or his Uptilt, leading to a kill with the latter, or kicking Sonic back offstage again (or ending with a kill) with the former two, repeating the process. Sonic has to be very clever when recovering so that he can force DDD to choose the wrong option, allowing himself to get back on ground and get back to running. It may be one of the few times where halting Sonic's descent with Dair and the Homing Attack can prove to be viable at times.
Grab destroys Spin Dash approaches easily and leads to pain, but DDD is so large that Spin Dashes + followups can rack up high amounts of damage regardless. DDD's ftilt is his longest ground move and is used for spacing, but I think Sonic can brake right outside its range, and Spin right at DDD before he can do anything after the move.
DDD's Bair is insane, they know it, we know it, everyone knows it. It has no lag to speak of and it has fantastic priority and range. Fair is slow-ish, but covers an enormous arc and is a kill move. I can't really comment much on Dair and Uair, but I think they can be DIed? Nair....isn't really used much, except maybe as a fastfall option.
DDD has either the slowest or the 2nd slowest overall horizontal air speed - Luigi's might be slower. And with Fair being on the slow side, Sonic can harass DDD with his own Fair and eat DDD's jumps (watch out for Inhale!). DDD's UpB does not have a hitbox as he is rising up, giving Sonic a bit of leeway to intercept it, but DDD can cancel the UpB at any point and descend immediately, so if the ledge is an option to grab, the DDD will likely go for it.
As far as special attacks go offensively, DDDs will be using Inhale and Waddle Dee Toss. Inhale is typically used when DDD is falling back to the ground so it can fight a land-sharking opponent. Inhale is also weird in that DDD can immediately Utilt characters if he decides to let them break out. I've seen videos where it works on Meta Knight, but does it work on Sonic too? As for WDT, They are easily powershielded, but be on the lookout for Gordos. Waddle Dees are mostly harmless, but Waddle Doos can prove to be a nuisance if left unchecked; moves refresh when hitting the Waddles, so there's little reason for Sonic not to knock them away with his ftilt or his Jab combo.
DDDs will be killing with Utilt, Bair if it's fresh, Dsmash, Fair, and most importantly, the new Dthrow - Usmash combo. The last one is daunting, as before Sonic was capable of avoiding DDD's kill moves easy, but now, Sonic will die at 130 to a grab. Trent mentioned before that it is wise to just stick to the edges in the MU so that Sonic won't take that much damage from the CG, but now it seems that idea is more important than ever at kill percents to avoid that Usmash.
I'm unsure about a verdict for the MU, but with that new CG-> Usmash present and omnipresent in the match, I can't see it being better than a slight disadvantage for Sonic.
