They're great if your frame only has one mounting point for a bottle cage and you already have your pump mounted to that spot. My fixed gear bike has a single bottle cage mount on the downtube so my pump goes there while i use one of these on the back:

http://trisports.com/midocasamo.html

The best thing is that there's still room for my saddle pack.

My recent bike build has two bottle cage mounts but since it's my long distance bike i still put the double saddle mount on it for the extra fluid capacity.