[prknight]

+1

not to mention that some people just do better when they have more than one project running concurrently.

A contrary approach to working non-parallel is the little and often philosophy (Mark Forster talks about this in his time management books). Working on big projects in small chunks, but with great regularity.

Personally I only switch to a strictly 'serial' approach when I'm faced with a deadline, I'm in a zone or when I'm feeling stubborn about getting something solved. The problem with serial mode is that it tends to reduce quality and drains energy to an extent.