A lot has been said about OpenSocial and Facebook, this is an attempt to say a bit more. As usual, a free-flowing list of observations:

• Google uses Javascript as its base client-side language and extends gadgets with OpenSocial features. This I consider “good enough” but perhaps more importantly has a much lower barrier to adoption. The way a FB app is delivered (request-mapping, restricted FBML, fine-grained privilege control) feels much more professional, but at the same time a bit intimidating for newbies to the field. Libraries for almost all languages (esp. rfacebook) lower the barrier, but it’s never as simple as a .xml file which is mostly html and javascript (this is what a gadget is).
• It’s currently not clear how OpenSocial apps communicate between each other across networks and how one app maps a user profile in one network to another. Without this, it’s mostly a single API which works across multiple containers in isolation. A single API that collates multiple containers is much more interesting.
• Is there any set of UI guidelines? Is a gadget supposed to emulate the look and feel of its container or is it supposed to look the same across networks? Facebook provides extensive CSS classes and Javascript goodies (AJAX friend selector) to keep the experience uniform. I hope OpenSocial addresses this and soon.