Does anybody actually know how strong Yuchiru is?? I don't see a big deal in her!

I don't believe the anime ever actually described or showed her abilities. She does have a Zanpakuto, though. For all we know, she could have a particularly devastating technique, though I somewhat doubt it. But, yes, she is very fast; not sure about her proficiency with Shunpo, though. I think she is only fast at running, not so much good at using Shunpo.

We know that she's no weaker than any other vice-captain, as many seated squad members, including Arimaki, show some fear and respect for her. Also, she spends a lot of time with Kenpachi, able to withstand his spiritual pressure no problem, as well as Byakuyas, Ichigo's bankai, and Nnoitora's ressureccion, all without even blinking. So we can assume she's strong, though he haven't seen her fight yet.