At work, we just shipped our first major new chunk of UI in a couple of years, written 100% in Script#. We've been watching Script# for a few years as an interesting option for creating client-side UI, and it recently hit a level of functionality where we felt it was workable. It also coincided nicely with our need for a mobile UI (a standalone product that we could roll out slowly, low-risk compared to changing our bread-and-butter desktop UI).
A little history
V2: all JS
Fast-forward to summer of 2009, when we shipped a 25k+ line all-JS jQuery client UI rewrite (which also included an all-new WCF webHttpBinding/JSON service layer). While it was originally planned to be testable via Selenium, JSUnit, and others, things were changing too fast and the tests got dropped, so it was months of tedious manual QA to get it out the door. User reception of the new UI was very warm, and we iterated quickly to add new features. However, refactoring the new JS-based UI was extremely painful due to the lack of metadata. We mostly relied on decent structure and "Ctrl-f/Ctrl-h" when we needed to propagate a service contract change into the client. Workable, but really painful to test changes, and there were inevitably bugs that would slip through when someone did something "special" in the client code. It got to a point where we were basically afraid of the codebase, since we couldn't refactor or adjust without significant testing pain, so the client codebase stagnated somewhat.
On to Script#
We'd been watching our user-agent strings trend mobile for awhile, and this summer it finally reached a point where we needed to own a mobile UI. Our mobile story to this point consisted of users running the main browser UI on mobile devices with varying degrees of success (and a LOT of zooming), and an iPhone app that a customer wrote by reverse-engineering our JSON (we later helped him out by providing the service contracts, since he was filling a void we weren't). The question came to how would we build a new mobile UI? Bolting it to our existing JS client wasn't attractive to anyone, as it's grown unwieldy and scary, and we didn't want to risk destabilizing it with a bunch of new mobile-only code. The prospect of another mass of JS code wasn't attractive to anyone. Another ECM architect (Matt Clay) had been watching Script# for quite awhile, and it had just recently added VS2010 integrated code editing (used to be a standalone non-intellisense editor that ran inside VS2008) and C# 2.0 feature support (generics, anonymous methods). These features gave us enough pause to take another look, and after a week or so of experimentation, we decided to try and ship the mobile UI written with Script#. I'll post something soon that describes what the end result looks like.