What is a best way to customize Bootstrap3 in Rails?

What is a best way to customize Bootstrap3 in Rails?

Twitter Bootstrap 3 has been released recently. We're going to use in a
new project and customize it to fit our needs.
Here're a few issues we came across in the past 2 projects.
as TB is written with LESS it will took some time unless there is a
reliable SASS version
working with unstable SASS version we found out that styles were imported
more than once - this is perhaps due to the old SASS bug that includes the
same files more than once
we could't make 'include-once' workaround work with SASS version from the gem
we figured out that the best what we can do is build TB from the builder but…
it requires a lot of work on our part just to do simple modifications like
e.g customizing navbar as we can't just override the default variables
We are looking for a solution that would make customizing Bootstrap less
problematic. What is your experience? What strategy do you use? Have you
tried to use TB3 in Rails(4)?