I wouldn't use a seatpost rack with much of a load - certainly not with 50 lbs. I read a report of a guy who used one for a medium load and he had problems with it swinging from side to side. I think a seatpost rack is a good around-town solution. I have one on my mountain bike and I use it to carry a tube, mini-tool, sandwich, jacket, book.

There are racks that can be mounted on any bike. Look at Old Man Mountain if you have discs. Tubus has adapters that allow you to mount racks on frames with no eyelets. Jandd makes quality racks.