I am running XP SP2 RC1 without any problem.
I used to run Tablet PC2004 Alpha prior to installing SP2RC1, with some minor bugs only.
This RC1 seems very solid. No problem with download speed either, running cable modem WIFI at home and T1 at work. No noticeable difference. MindManager X5 Pro is the only program affected by this install- the pen mode doesn't work!