I used the time-speed up heaps. I quite liked it. I mean it made everything just that little bit harder, and it really upped the pacing!
That is a cheat I would like to see return.
For me, using the time speed up cheat just caused the game to glitch a whole lot - for example, during the final boss, I died so quickly that the game thought I was still in the arena in the centre of the Clock when in fact I'd respawned outside - meaning that I couldn't return to the boss fight because the area was locked off to prevent me from escaping the arena that I wasn't even in!
The only way I could fix it was to enter the code to reset challenge mode and start all the way back from Quantos…
That happened to me too. I fixed it somehow, and I didn't need to replay the whole thing.
I used the time cheat for the first ten round if the final challenge in Battleplex when I was going for the Flawless skill point.