Mega speedup you mean?

To be honest mega start sort of negates its usefulness but I think it's handy to have for stages where it isn't worth buying mega start or the stage doesn't offer it. Plus it being permanent is nice.
I'm at the end of the game with near max coins so there's nothing left for me to do in shuffle so for me it's worth it as it gives me a reason to play it by farming coins getting to near max again. I've tried the stage twice now with items and I'm at 85k or so coins still and will get back to ~95k before the next set of stages come out easily. It really just depends, I didn't go for the 100 stage Giratina Speedup as it was a little too difficult to get to and if they ever make it too hard to get more I probably won't actively pursue getting them all but if I can get them without much trouble, like this one then I will. So I don't think they are too useful and I won't go out my way to get every last one but they are nice to have if you can get them without much hassle.