Holy hell I wish this existed when I wrote addons for Safari, Chrome, and Firefox. Just the idea of IE support with no extra work is awesome - and the idea of having a single codebase is great too. I tried to modularize as many components of my addons as possible to make them all easy to update but it just wasn't perfect. I went as far as building a messaging class to extend for the communication across platforms and boy was that a hack and a half. DEFINITELY trying this out next time I'm building an addon!
Building browser extensions with trigger's tools is really straightforward if you're familiar with Chrome/Safari/Opera/...'s extension api.
The most surprising thing for us was that IE extension mostly(3 lines of IE specific code) just worked.(Before that we had a huge black box of code that nobody dared to touch.)
I'd love to write a long post about our experience but there is nothing special to say. It works and it's great.
Been using crossrider for the last couple of months. They've been doing a pretty good job as well(yup, not having to write the IE plugin is an absolute godsend!). Good to have an alternative though, I'll definitely be taking a look at trigger.io