We've written lots of posts about pack-rafts and pack-rafting in the past and have no intention of recreating the wheel - instead, i've just re-linked to them so that those newbies to ENDracing's brand of adventure racing can so some reading to know what to expect during the optional (but so totally awesome) pack-rafting segment of the upcoming spring race. In addition, i wanted to remind everyone (i've been told by a number of years that they are buying proper pack-rafts this year) that we can order Alpacka Rafts - the top end of packrafting from Alpacka for 10% off. You don't need Alpacka rafts to enjoy pack-rafting in our races (a team in 2011 used an inflatable air mattress!), but most other boats will wear out after a number of uses, and do require more care during use in an AR type setting. In END-SPAR you will need to plan on carrying whatever you decide to use for the pack-raft section for a little less than a mile on foot.
Alpacka boats cost about $500 for the ideal AR version (the scout). Lower end pack-rafts can be found at discount super stores like Walmart or on Amazon for as little as $10 (search "intex boat"). Happy shopping!