We are currently reviewing XForms for a project. There are several small, active projects previously mentioned, although Ubiquity has been merged into backplanejs, and that appears moribund.
Orbeon is certainly actively developing and releasing new versions, and slowly adding features from XForms 2.0 standard. From an external point of view, they appear to be the healthiest and are LGPL/commercial. They are not a pure client solution, though, as there is a server component.
BetterFORM is also actively developing and releasing new versions, and their solution is pure client-side for XForms.
There are a couple of small projects that look interesting: XSLTForms seems to be the liveliest and is 100% browser-based; there's a dual license LGPL/commercial.
In my opinion, it's a great technology but it's also a niche technology. The ability to "externalize" chunks of your user interface is extremely useful (especially in my company's application), but it doesn't result in UIs that are as good as the best HTML5 UIs.
I do not know whether client-only XForms support is the best approach in an age of apps and mobile devices. I do know that I'd like to see a product that produced UIs compatible with popular Javascript frameworks like extjs to simplify inclusion of XForms into an existing UI.
If you need it, it's out there, but it doesn't look like it will ever be big.