In my (limited!) experience they're good for a couple of stops improvement over hand-holding, maybe three at a pinch, more if you can dig the 'foot' into the ground and crossbrace them with a convenient tree/rock/fencepost.

Another thing you might want to additionally consider is a beanbag - they do them now with a tripod screw fitting on them for a bit more in the way of security. Again, you need to have a convenient tree/rock/fencepost...!