pptp secure?

I got a concentrator 3000 and about 20 or so remote users who vpn in using pptp for win xp/2k rc4 128 bit. How secure is this? I am thinking about moving to the cisco vpn client at 3DES 168 obviously this would be more secure but less easy to use for users. I have read various articles most say so long as it is XP/2k it is ok. Any opinions would be greatly apperciated