When I was evaluating this gem, I wondered if I could use it for all my client_side validations. I've seen threads on Railscasts, the simple_form Google Group, etc where people wonder the same thing.
We all want the goodness of c_s_v, but nothing is worse than having multiple validation engines dueling away in the UI (so I'm glad to see you are working on a backbone.js plugin).
To allay these fears, it would be nice if the README documented a few more custom validation use cases. Forex, I wanted to see a pattern for validations that depend on multiple attributes to fire when either field changes. I extracted my solution from: https://github.com/bcardarella/client_side_validations/wiki/Shared-validations-for-multiple-fields
.change(-> $('#first_name').change(); true)
.focusout(-> $('#first_name').focusout(); true)
My validation decorates #first_name with the error, but now it fires when the other fields are modified or visited. This may seem obvious to some, but it took me a while to figure it out. I think if more people knew up-front (on the README) how comprehensive this gem was for all your validation needs, they would dive in faster. I wish I had done it sooner.
Just my 2 cents.
Perhaps the README could link to a Custom Validations wiki page like you did for the Plug Ins and the community could add some examples like the multiple_fields example above and the one I proposed.
This approach also works very nice with backbone.js
class MyApp.Views.Person extends Backbone.View
'change #person_last_name' : -> $('#person_first_name').change(); true
'change #person_company' : -> $('#person_first_name').change(); true
'focusout #person_last_name' : -> $('#person_first_name').focusout(); true
'focusout #person_company' : -> $('#person_first_name').focusout(); true
Since this was coffee script (which is opinionated about return values), I needed to return true to continue event bubbling