How good is it for SEO if you have a widget that lives on other sites?

That widget will do you no good as it is a frame which is not SEO friendly. So the content inside the widget is essentially invisible to the search engines. Additionally, there is no link to your website in that HTML so you're not getting any links from that widget anyway. Basically this is not how you want to make a widget.

You should have that widget be powered by JavaScript and use the <noscript> tag to add alternate content that works if the JavaScript is not available for that user. In that content you should link back to your website.

For example:

<script src="http://example.com/widgets/widget.js"></script>
<noscript>
  <iframe src="http://example.com/widgets/iframe.php">
    <img src="http://example.com/widgets/placeholderimg" width="200" height="200" alt="Map Widget">
  </iframe>
  <p><a href="http://example.com/">This widget provided by example.com</a></p>
</noscript>

That example uses graceful degradation (JavaSCript > iframe > image > image alt text) so the content is available to everyone (good accessibility) and does not use any black hat techniques while still getting a real link to your website.