I have no experience with JavaScriptMVC, but here goes my two cents: My background is mostly C++/Java nonetheless I have been doing web programming for a year now, mostly actionscript, but also server side java, and javascript, css, and so on. I've been initiated in a Javascript/DWR project recently and I'm using PureMVC, which I already use and like on my works on flex/actionscript. PureMVC is the single most popular mvc for flex/actionscript btw. It also has been ported for dozens of languages and since its conception had a language agnostic approach in mind.
You might be aware of how similar the syntaxes of actionscript 3 and javascript are, so it's really easy to jump from one thing to the other. Therefore I think that would be one point in favor for you checking PureMVC out.