Please read for an updated status on RailsCasts:
Learn more or hide this
GitHub User: stmontgomery
FYI, I was able to have the same styling on the f.button when omitting 'btw' from :class, since 'btn' is one of the default classes the Twitter Bootstrap-powered version of SimpleForm has (defined in config/initializers/simple_form.rb):
<%= f.submit nil, :class => 'btn-primary' %>
Can leave out 'btn' from :class because its specified here:
# Default class for buttons
config.button_class = 'btn'