This from Wikipedia again:

"Picaridin, first used in Europe in 2001, has been reported to be effective by Consumer Reports (7% solution)[6] and the Australian Army (20% solution).[7] Consumer Reports retests in 2006 gave as result that a 7% solution of picaridin offered little or no protection against Aedes mosquitoes (vector of dengue fever) and a protection time of about 2.5 hours against Culex (vector of West Nile virus), while a 15% solution was good for about one hour against Aedes and 4.8 hours against Culex.[8] Natrapel 8 hour is a 20% based picaridin repellent that claims to be effective for 8 hours."

While everyone must make his or her own decision, for me as a big sweater, I much rather treat my clothes once before I leave for the trip rather than having to apply the stuff directly to my skin three times a day.