A local machinery rebuilder told me to get one of these sets years ago and showed me why -- the arms are straight, so you can grab your favorite clamp and clamp them together. This way the arms don't slip off of the thing you're pulling. I have seen a lot of castings with broken off chunks around the edges where puller jaws started to splay out and put too much pressure on the rim or the workpiece. Not a problem with this set/method.
These Proto/Plomb pullers (along with Posilok and Crane) are my favorites for this reason.
Pro-tip -- on non-critical stuff where you're less worried about the arms splaying, put a rubber band around them to hold them together. It makes it much easier to get things started without one arm slipping off.