I like the idea of microformats but as a visitor to a website they're not useful to me. Only until I dig into the source I see the microformat code. Is there a web browser which doesn't require a plugin that parses these microformats?

As a developer they create a mess and cause excessive namespaces. Take this example from wikipedia:

<div class="vcard">
<div class="fn">Joe Doe</div>
<div class="org">The Example Company</div>
<div class="tel">604-555-1234</div>
<a class="url" href="http://example.com/">http://example.com/</a>
</div>

Is this microformat expandable? If I have two phone numbers how do I identify which one is office, personal and mobile? Why do I have to use the class attribute - this is for style. Even if I changed them to use rel this is also bad.

I would use them when it has been implemented correctly.

HTML and XHTML was not designed for microformats. It's a misuse of HTML.