I've been using Protovis, and my main complaint about D3 is that it brings you closer to the DOM. I suppose that's the point, and is generally a strength, but it doesn't give you a nice abstraction layer for cross-vector-language bindings (svg vs vml). Things like SVGWeb help where SVG isn't available, but it's not quite as nice as direct VML support.
Can't wait for full SVG support in all common browsers!
I launched a small site using D3 (windhistory.com). It's great. SVG is now supported in Firefox, Chrome, IE, and Mobile Safari. The big limitation is IE7 and IE8, but if you can blow those users off SVG is fantastic.
Unfortunately, I can't yet blow those users off. The last set of visualizations I did with Protovis saw over 20% of its traffic in IE < 9. I'd be ecstatic if I could ignore VML...
Your site breaks the back button worse than any site I've ever encountered. I found that to be incredibly annoying, but I think I would have liked the site if it weren't for that.
I love how low-level D3 is. My biggest issue with other visualization libraries is constantly wondering, "can I do this?", or searching around for specific features, but with D3, since all you're doing is manipulating the DOM with some data, you can do whatever charts and visualizations your imagination can conjure up.
It took me three days of intense study at the JavaScript console, but I managed to build some seriously cool dynamic data visualizations for text comparison (that I unfortunately can't show off yet). Anyway. Use this library, it's killer.
Can't wait for full SVG support in all common browsers!