I find these words in pptp client source :
by Peter Mueller
PPTP is known to be a faulty protocol. The designers of the protocol, Microsoft, recommend not to use it due to the inherent risks. Lots of people use PPTP anyway due to ease of use, but that doesn't mean it is any less hazardous. The maintainers of PPTP Client and Poptop recommend using OpenVPN (SSL based) or IPSec instead.
(Posted on 2005-08-10 to the mailing list)
But as far as i know, there are many people use PPTP as a VPN, because there is no need to install client on windows, what do you think about pptp ?