What are peoples experience with limb spacers?
Which work best for young fruit trees?
I have even seen ones that seem to pierce the tree (toothpicks) but this doesnt seem like a good idea to me.
You may mean limb spreaders. I have used these on young trees--under 7 years old--to prevent the laterals from taking a more vertical position. Sometimes hanging weights from the limb works better. I have made my own limb spreaders from pieces of wood approx 1"x1" and as long as needed, usually about 18". I tried cutting a V in the ends to secure them in position, but that did not work. What works better is to make sort of a metal "toothpick" as you say, by driving a nail into each end, then clipping off and sharpening with a grinder. This makes a small wound in the tree, but nothing that causes a problem.
For very young trees, you should be using clothespins to create a good crotch angle on a lateral right from the start.
The African version is to take pruning cuttings about 1/2" and up caliper, cut each end at a steep angle with your pruning shears, and then cut a V notch in the tip of each angle. They need to be braced against a bud, but they do stay put in winds.
If you manage a lot of trees, tree spreaders such as the Treform design are invaluable. The wounds created even by spreaders made with two-pointed nails are not a problem as they heal once the spreaders are removed. Being able to cut through the bark helps the spreader stay in place and to get the exact angle you want.
I don't like the imprecision of weights and only use them when I have to ( and only when renovating very large apple trees and spreading large branches). You may think the weight is just right but then the branch grows and the wind comes up and you may have a broken branch. I end up erring on the side of caution and don't get as wide an angle as I want. Better to use a stake and string. Spreaders are much quicker and you don't have to mow around them.
I don't use toothpicks or clothespins because I wait for branches to get larger to spread. It is an affective way to train trees though, except when its a variety whose growth habit will send the direction straight up once the branch gets beyond the push of these small spreaders. For Northern Spy and the like (and most pears) you need 2' spreaders to have any significant affect.
I have used whatever I have hanging around. I mulch alot so I have plenty of space under my trees for using string attached to rocks on the ground. Sometimes I have Stakes or a piece of heavy wood with a nail attached to string. On my row of tall spindle apple trees I have enough limbs near each other that I tie them to the other limbs to shape them down.
If prunings are used as spreaders, can disease (fireblight or other) get started in the severed branches and then spread to the remaining tree?