I'm also a 30' guy ... like JB said, big enough to fly shapes or patterns, but short enough to get the job done.
My exception (space not withstanding) is that horrible circumstance where the 'wind' is constantly shifting directions (usually breathing back and forth with gusts from random directions in between) and I need to be able to do a 180 (horizontal or up and over) in a heartbeat because I'm suddenly on the wrong side of the window. In this case I am currently developing my skills on 12' lines, but my intention is to be able to stretch back out to 30' lines.
The two instances where I think 12' works are near zero wind (check Youtube for JB rev'ing it in Singapore) and/or when you interact with the environment to make it interesting (check Youtube for JB in the UK).
A good contrast can be seen in Watty's 'Friendly Neighbourhood Street Flyer' video