Depends on the size of your Gemfile. There will inevitably be gems that have unpatched Rails 4 bugs.
Really, it's not large. The things I'm most worried about are
gem 'carrierwave'
gem 'jquery-fileupload-rails'
gem 'jquery-datatables-rails'
gem 'jquery-ui-rails'
I don't have a good answer: Fine to program with, but certainly took some doing for our particular need. (We had a number of requirements that made the "easy" out-of-the-box jquery-fileupload not suitable.)
Ask me in a couple of months and I'll probably have a lot of opinions to give.
> and when I saw there was some weird iframe thing for IE... I closed the page and walked away.
We are migrating customers from a C++ desktop app to a web interface: So anyone on old IE with problems will be informed that they're not supported on the new system, so keep using the old and let us know after they upgrade. :) (Newer IE is OK.)
Deviously, this also prevents absolutely everyone from ditching the old app and swamping the new system on day 1.
The iframe transport method for uploads is useful for browsers that don't support XHR file uploads (basically IE < 10). You can read more a bit about how it works in: https://github.com/blueimp/jQuery-File-Upload/wiki/Frequentl... or directly in the source: https://github.com/blueimp/jQuery-File-Upload/blob/master/js... (it's pretty small).
You'll have a bit of work to do to migrate to strong parameters, but you can always add the protected_attributes gem back to re-enable support for attr_accessible.
I'd suggest checking out the Rails 4 migration RailsCast.