I think very rare.Its a nice CMS and have tried myself.It has many good features from the backend.Still not easy as Wordpress.
I use MODx a lot. It's excellent although it does need a little getting used to when setting things up. The amount of control is amazing. I prefer the Evolution form to the newer Revolution, which I find is a bit slow.
we use MODX alot for all types of website development from simple small business sites right up to complex corporate sites. Our developers and clients love it. Some 'big' site examples include biffa.co.uk, atseuromaster.co.uk and theloughborough.co.uk It's flexible, quick to develop with and easy to use from a client admin perspective. Down sides are a current lack of good extensions and the specific hosting requirements has also caused us a few issues with some hosts.