This fork adds a custom horizontal form wrapper and merges append/prepend in a 'group' wrapper

collection_check_boxes_input.rb

#app/inputs/collection_check_boxes_input.rb
class CollectionCheckBoxesInput < SimpleForm::Inputs::CollectionCheckBoxesInput
  def item_wrapper_class
    "checkbox-inline"
  end
end

_form.html.haml

#radio buttons with a column width override for the 'right_column' wrapper
= simple_form_for(user, wrapper: :bootstrap3_horizontal, defaults: {right_column_html: {class: "col-lg-3 col-md-3"},
  label_html: {class: "col-lg-2 col-md-2"} }, html: {class: "form-horizontal", role: "form"}) do |f|
  = f.input :email
  = f.association :roles, as: :check_boxes, right_column_html: { class: "col-lg-10 col-md-10" }

collection_radio_buttons_input.rb

#app/inputs/collection_radio_buttons_input.rb
class CollectionRadioButtonsInput < SimpleForm::Inputs::CollectionRadioButtonsInput
  def item_wrapper_class
    "radio-inline"
  end
end

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 :bootstrap3_horizontal, tag: 'div', class: 'form-group', error_class: 'has-error',
      defaults: { input_html: { class: 'default-class '}, wrapper_html: { class: "col-lg-10 col-md-10"} } do |b|

    b.use :html5
    b.use :min_max
    b.use :maxlength
    b.use :placeholder

    b.optional :pattern
    b.optional :readonly

    b.use :label
    b.wrapper :right_column, tag: :div do |component|
      component.use :input
    end
    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 :group, 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
    b.use :input, wrap_with: { class: "input-group" }
    b.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
    b.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
  end

  config.default_wrapper = :bootstrap3
end

It's better to include :hint and :error into right_column wrapper for :bootstrap3_horizontal:

b.use :label
b.wrapper :right_column, tag: :div do |component|
  component.use :input
  component.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
  component.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
end

Else, hints and errors will not appear under input with right offset.

Rather than overwriting item_wrapper_class to always use the inline version, I'd recommend:

class CollectionCheckBoxesInput < SimpleForm::Inputs::CollectionCheckBoxesInput
  def item_wrapper_class
    nested_boolean_style? ? "checkbox-inline" : super
  end
end

This way, you can set :boolean_style to something else than :nested to render checkbox collections (as done by simple_form's association for instance).

Creating top-level objects TextInput etc doesn't play nicely when using simple_form and formtastic in one project. So I created a solution that directly overrides simple form's internal mappings (which is a bad hack because these APIs might change):

inputs = %w[
  CollectionSelectInput
  DateTimeInput
  FileInput
  GroupedCollectionSelectInput
  NumericInput
  PasswordInput
  RangeInput
  StringInput
  TextInput
]

# Instead of creating top-level custom input classes like TextInput, we wrap it into a module and override
# mapping in SimpleForm::FormBuilder directly
#
SimpleFormBootstrapInputs = Module.new
inputs.each do |input_type|
  superclass = "SimpleForm::Inputs::#{input_type}".constantize

  new_class = SimpleFormBootstrapInputs.const_set(input_type, Class.new(superclass) do
    def input_html_classes
      super.push('form-control')
    end
  end)

  # Now override existing usages of superclass with new_class
  SimpleForm::FormBuilder.mappings.each do |(type, target_class)|
    if target_class == superclass
      SimpleForm::FormBuilder.map_type(type, to: new_class)
    end
  end
end

I had to change the group wrapper to the following so that you can set its right_column html:

  config.wrappers :group, 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
    b.wrapper :right_column, tag: :div do |component|
      component.use :input, wrap_with: { class: "input-group" }
      component.use :hint,  wrap_with: { tag: 'span', class: 'help-block' }
      component.use :error, wrap_with: { tag: 'span', class: 'help-block has-error' }
    end
  end

Thanks! I use form horizontal a lot and this has helped!

Has anyone managed to fix the label class issue?
I have set this in the initializer:
config.label_class = 'control-label'
I don't want to set this to 'control-label col-sm-2' because it will mess up all the other forms that are not horizontal.

@anthonykaras you just have that in example

defaults: { right_column_html: { class: 'col-xs-6' }, label_html: {class: 'col-xs-2'}}