Last time I played around with SilverStripe, it seemed like a pretty solid CMS (outputs valid XHTML, easily templatable, etc.) and I probably would have used it in a production site I was doing at the time, but I had trouble with the WYSIWYG javascript file uploader in Safari, which was the browser of choice for the client.

I imagine they’ve fixed the issue now though, so if you’re a developer and looking for alternatives to Joomla!/Drupal, check out SilverStripe.