A Google Webmaster Help thread has Google's John Mueller responding to some complex robots.txt questions.
In that, he strongly recommends that when you do set up a robots.txt file, make sure you keep it very simple. John said:
When possible, I'd really recommend keeping the robots.txt file as simple as possible, so that you don't have trouble with maintenance and that it's really only disallowing resources that are problematic when crawled (or when its content is indexed).
Heck, John even has recommended you remove the robots.txt file completely if it is not needed but always make sure to keep it under 500KB if you do have to go more complex.
To see the context this is in, see the Google Webmaster Help thread.
Personally, I am all for not using robots.txt files when possible.