At "On & On Creative":http://www.create-on.com we have a lot of sites on a common rails backend. It's incredibly flexible so we can offer a lot of support to customers but because there are so many customers with different needs testing has become a huge pain. Not only does it let us run tests custom to each client, it provides a way to import the data to the test db really easily and it's way faster than loading the fixtures each time you start a test. Yay!
After googling around I happened upon "this caboo.se article":http://blog.caboo.se/articles/2007/2/13/what-about-those-bloated-tests about how to run without fixtures at all. It's a very indepth article with lots of good things in it; far be it for me to try to compete.
Instead, here's the low down on how we prepare each of our sites for testing.
Add the following to the end of your rails_root Rakefile:
Add the following to lib/tasks/fixtures.rake. Make sure to look at the two commented lines:
Then the fun easy stuff happens:
If you want to commit the fixtures to the repo you can but it seems a waste to me since you might be regenerating them often.
Also, check the two comments in the fixtures.rake, it could be doing things you don't expect.