Just finished CTDS earlier this afternoon; it was all I expected and more.
The new translation was great. It made the game feel more mature and polished, while still maintaining the lightheartedness of the original translation. My only complaint is the "fiend" thing. I don't get why they can't say "demon."
Maybe my memory is foggy on the PSX version, but the DS cutscenes seemed to be implemented more smoothly. I recall the PSX scenes being used awkwardly, like they were more of distraction. The DS version seemed more fluid, and of course, the elimination of the graphical lag were a Godsend.
For the most part, the touch screen features were a welcome addition. Being able to point to the actions you want to execute instead of scrolling through was accurate and quick. Moving with the touch screen however... was iffy. It wasn't awful, but it could be spastic at times, and with the d-pad being so much tighter and easier, there's little reason to use the touch-screen movement unless you really just want to try something different.
The Lost Sanctum was mildly annoying. Many of the quests were either recycled from other sidequests in the game or are annoying fetch quests. The rewards were worth it, though, and the ultimate outcome of all the work was pleasing. Still, the very existence of the Lost Sanctum seemed out of place. Though since it's a sort of spacial distortion, I guess it's kind of supposed to be that way.
The Dimensional Vortex was much more interesting and more relevent. Storywise, it raised some questions, but it also answered some. Gameplaywise, it made for some very interesting dungeoncrawling, borrowing elements from earlier areas while introducing some new ones,
The optional boss was a nice extra challenge and also raised and answered some story questions. After a month of waiting for CTDS and avoiding message threads with the title "Optional Boss Fight!*SPOILERS*," even though I suspected all along who it was(and was correct), finally seeing the optional boss was quite a rush. The fight itself was... I think "short and sweet" describes it accurately. As opposed to the endurance-like battle with Lavos, the optional boss is much faster paced and more threatening. Then it's over as suddenly as it begins.
I'd like to comment on the Arena of Ages, but I've yet to do much with it. I've fought a few battles, and it's mildly entertaining. Ultimately, it doesn't seem worth it. I can see it being fun playing against human players, but CPU opponents leave something to be desired.
All in all, great additions to an already great game.