I haven't followed Givewell recently, but they were involved in a fiasco over at metafilter where they were sockpuppeting to promote themselves and bad-mouth competitors. Unethical behavior of the founders aside, the devil is in the details and you should do your homework before falling in love with their elevator pitch.

http://metatalk.metafilter.com/15547/GiveWell-or-Give-em-Hel... is the main thread, but search metatalk for several follow-up threads.