At my school we have both fall and spring rush and spring rush is always the most successful. This is because we do deferred rush. You cannot pledge your first semester (for traditional/non-transfer students), so most people rush the second semester freshman year. That's when I did. Then the fall is generally the third semester overall (first semester sophomores) who liked the idea esp. after their friends went through it. This system works really well here because we are small (1500 kids) and you get acclimated before you pledge.
I would reccomend deferred rush to most schools.
