If you haven't committed to the seatpost rack, don't. They are great as a last resort, IMHO, but if your bike can accomodate a standard rear rack you're better off using one.

Even if you don't have rack bosses or eyelets, all hope is not lost.

http://www.thetouringstore.com/TUBUS...ONS%20PAGE.htm

I used a seatpost rack on a race-oriented road bike I had, and no matter what I did it tended to drift when I leaned. Of course, it was a Topeak rack with a quick release, so that may have made a difference. Still, running panniers is much easier with a conventional rack.