Do you think Z-Moves will be viable competitively?

In the one hand, having a seemingly extremely strong fifth move sounds great, especially for those sweepers who suffer from the 4 moveslot syndrome.

However, the fact that you need an special item on top of that you can only use your z-move once per battle definitely turns it into a high risk-high reward matter.

What pokemon do you think will find them the most useful, if any at all?