It looks like quite a cool idea, but a cursory google search finds nothing but code snippets: no "large" projects that are made purely using WebSharper, that would serve as a sanity check "this really works". Any experience playing around with it for a somewhat extensive period of time (i.e. to make a blog or personal website) would also be useful.
In theory it sounds like a wonderful idea: compiling a single language to multiple things to run on client, server, etc.. Typically though, the abstraction is leaky and/or restrictive enough to make it just as annoying as writing the multiple languages by hand (that's my experience with GWT) and was wondering if WebSharper was any different.
As creators of WebSharper we have used it internally quite a bit, the largest project to date being FPish (http://fpish.net). Our clients use WebSharper for large projects as well, this work includes an enterprise online data visualization solution. So yes, it passes the sanity check.
That being said, there are areas where WebSharper could do a better job to support larger projects, such as compilation speed, and we are working to address those.
You make an interesting point about the restrictive or leaky abstractions. I have not used GWT, so I cannot comment much on that. Because of its functional type system I find F# to be vastly superior to both Java and JavaScript, which makes me more productive using WebSharper. Switching to F# can be a hard sell though.