I've configured VPDN for PPTP on a Cisco router before and it works well as a Microsoft PPTP server, either for local authentication or to a Radius server. It makes a decent replacement to a real NT server for PPTP access in my limited trial.
However, I have not load tested it. With just a single user session, I managed to make the CPU on a 1751 router go to 50% and on a 3725 router, it hit 16% when moving about a T1 of bandwidth. That doesn't lead me to believe it will scale very well.
Has anyone had any success with 50 or 100 PPTP users on a Cisco router, of any model or size?