I once drove 10 hours to visit a friend but my phone was dead on arrival. I had his address but no apartment number. I used the Kindle experimental browser to message him on facebook.
The big problem with the Experimental Browser on my beloved Kindle DX is that it doesn't seem to support tls/https.
Otherwise I'd use it a lot more.
It's been probably a couple of years, but I tried to load HN in it once and it was unreadable, so I don't think it's too great.
I read HN on my Kindle's browser fairly regularly. HN itself (and similar pages with a simple layout, such as i.reddit.com) works great. For the articles that are linked, it's a coin flip whether it works or the page has so much cruft that it crashes the browser.
I used to use the built-in browser to load Google Voice and Google Talk and they worked fine. Of course, this was before they were overhauled into bloated, JS-heavy pages that barely run on non-Chrome mobile browsers.