I've tried both RC1 and RC2 so far, and both just about installed (once I reduced the amount of RAM from 2GB to 1GB - strange!) but the biggest show-stopper as far as I'm concerned is the lack of driver support by vendors.
Come on Abit (Highpoint RAID drivers please!), EMU (ASIO2 driver for my soundcard please!). Also some decent bus-layer drivers to allow direct CD access wouldn't go amiss - otherwise iTunes cannot read/burn CD's...
Press release about the RTM here: http://www.microsoft.com/presspass/press/2003/mar03/03-28WinXP64BitPR.asp