I\'m a huge fan of asp.net 4.5 webforms model binding using data annotations.
ascx:
As we have found in our ASP.NET WebForms projects, there is no overall useful reuse of the Model's validation attributes for client side validation.
For example, a contact data model, with various properties like name, email, birthday etc... is not always used the same way. Sometimes it may have some mandatory fields, sometimes not, and even the required input data may differ at various points in the application.
Thus, in our projects, we use both a client side validation implementation, and the model attributes.
The general idea we apply is:
For the client side, we have chosen the jQuery Validate Plugin (http://jqueryvalidation.org/).
We even have built our own set of controls (that derive from the built-in WebControls), which render various (and even some custom) data rules.