Forster's primary sizing dies (those with an expander ball) are very good, and I use several.
But aside from being well-made, the other reason they result in straighter case necks (and hence loaded rounds) is the expander ball's high up inside the die, just below the neck portion of the die. Thus the neck of the case is still inside the neck portion of the die when it's pulled back over the expander, so the case is forced to remain in-line with the die. This definitely results in straighter necks than standard sizing dies where the expander ball is down near the bottom of the case, where necks can pull out of alignment.
That said, if the case necks aren't very even in thickness the neck alignment can still be a little off even with Forster dies, but generally not more than .002"
Another reason I like Forsters is their honing service. About half my full length forster dies have been honed to my desired neck diameter. I usually take a skim cut with the neck turner then have the die honed to .003" less than a loaded round. I throw away the expander ball. I've had good luck with the honed dies.
