this is a simple form initializer that is ALMOST compatible with Bootstrap3 apart from a way to set the class of the label this would work perfectly

simple_form.rb

# http://stackoverflow.com/questions/14972253/simpleform-default-input-class
# https://github.com/plataformatec/simple_form/issues/316
 
inputs = %w[
  CollectionSelectInput
  DateTimeInput
  FileInput
  GroupedCollectionSelectInput
  NumericInput
  PasswordInput
  RangeInput
  StringInput
  TextInput
]
 
inputs.each do |input_type|
  superclass = "SimpleForm::Inputs::#{input_type}".constantize
 
  new_class = Class.new(superclass) do
    def input_html_classes
      super.push('form-control')
    end
  end
 
  Object.const_set(input_type, new_class)
end
 
# Use this setup block to configure all options available in SimpleForm.
SimpleForm.setup do |config|
  config.boolean_style = :nested
 
  config.wrappers :bootstrap3, tag: 'div', class: 'form-group', error_class: 'has-error',
      defaults: { input_html: { class: 'default_class' } } do |b|
    
    b.use :html5
    b.use :min_max
    b.use :maxlength
    b.use :placeholder
    
    b.optional :pattern
    b.optional :readonly
    
    b.use :label_input
    b.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
    b.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
  end
 
  config.wrappers :prepend, tag: 'div', class: 'form-group', error_class: 'has-error' do |b|
    b.use :html5
    b.use :placeholder
    b.wrapper tag: 'div', class: 'controls' do |input|
      input.wrapper tag: 'div', class: 'input-group' do |prepend|
		prepend.use :label , class: 'input-group-addon' ###Please note setting class here fro the label does not currently work (let me know if you know a workaround as this is the final hurdle)
        prepend.use :input
      end
      input.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
      input.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
    end
  end
 
  config.wrappers :append, tag: 'div', class: 'form-group', error_class: 'has-error' do |b|
    b.use :html5
    b.use :placeholder
    b.wrapper tag: 'div', class: 'controls' do |input|
      input.wrapper tag: 'div', class: 'input-group' do |prepend|
        prepend.use :input
		prepend.use :label , class: 'input-group-addon' ###Please note setting class here fro the label does not currently work (let me know if you know a workaround as this is the final hurdle)
      end
      input.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
      input.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
    end
  end
 
  # Wrappers for forms and inputs using the Twitter Bootstrap toolkit.
  # Check the Bootstrap docs (http://getbootstrap.com/)
  # to learn about the different styles for forms and inputs,
  # buttons and other elements.
  config.default_wrapper = :bootstrap3
end

I managed to add a custom class to a label. Like this:

class Labels < SimpleForm::Components::Labels
  def label_html_options
    label_html_classes = SimpleForm.additional_classes_for(:label) {
      [input_type, required_class, SimpleForm.label_class].compact
    }

    label_options = html_options_for(:label, label_html_classes)
    if options.key?(:input_html) && options[:input_html].key?(:id)
      label_options[:for] = options[:input_html][:id]
    end

    if options.key?(:label_html) && options[:label_html].key?(:class)
      label_options[:class] += options[:input_html][:class]
    end

    label_options
  end
end

# Then set the wrapper like this:

config.wrappers :bootstrap_horizontal, tag: 'div', class: 'form-group', error_class: 'has-error',
      defaults: { input_html: { class: 'default_class' }, label_html: { class: 'control-label' } }

Where would you put that code @Nerian ? Didn't work for me
What did work was just setting

config.label_class = 'control-label'

To get this to work with ClientSideValidations, I had to change 'bootstrap3' on lines 32 and 78 to just be 'bootstrap'. I believe something in the ClientSideValidations gem is making 'bootstrap' the default wrapper, so referencing it with the SimpleForm setup makes it work. Otherwise, I was getting a javascript error on validation of my fields.