user18478: The Vox Populi badge is described as :"Use the maximum 40 votes in a day".
However I tried to get this one a couple of times now and I never got to more than 31 votes. Then I get the message "You have used all your votes for today".
It looks like a bug to me or there should be a requirement to have "XX question votes up/down votes" or a rep of ZZ to get to 40 votes?  
As to the "usefulness of the badge"
Well ... how useful are the badges anyway :-s.
I see a benefit in "appreciating other peoples effort" by upvoting answers and questions (and somehow in downvoting as well). After all this is a questions and answers site and not a discussion forum.  
For those willing to give it a try. It is quite some work to find 40 questions/answers worthy to upvote in a day.   

user21270: You have 30 votes per day. This is the limit.
And in addition to that, you get an extra 10 votes on questions. Read this blog post to find out why.

You can vote 30 times per UTC day. You get an additional 10 votes on questions only.

I stumbled accross this little detail at the down votes privilege help page. So if you can only vote 31 times, try if you still can vote on questions.

18478: <p>The Vox Populi badge is described as :"Use the maximum 40 votes in a day".<br>
However I tried to get this one a couple of times now and I never got to more than 31 votes. Then I get the message "You have used all your votes for today".<br>
It looks like a bug to me or there should be a requirement to have "XX question votes up/down votes" or a rep of ZZ to get to 40 votes?  </p>

<p>As to the "usefulness of the badge"<br>
Well ... how useful are the badges anyway :-s.<br>
I see a benefit in "appreciating other peoples effort" by upvoting answers and questions (and somehow in downvoting as well). After all this is a questions and answers site and not a discussion forum.  </p>

<p>For those willing to give it a try. It is quite some work to find 40 questions/answers worthy to upvote in a day.   </p>
 21270: <p>You have 30 votes per day. This is the limit.</p>

<p>And in addition to that, you get an <strong>extra 10 votes</strong> on questions. <a href="http://blog.stackoverflow.com/2011/05/vote-for-this-question-or-the-kitten-gets-it/">Read this blog post to find out why</a>.</p>

<blockquote>
  <p>You can vote <strong>30</strong> times per UTC day. You get an additional <strong>10</strong> votes on questions only.</p>
</blockquote>

<p>I stumbled accross this little detail at the <a href="https://arduino.stackexchange.com/help/privileges/vote-down">down votes privilege</a> help page. So if you can only vote 31 times, try if you still can vote on questions.</p>
