I had some old servos given to me. I replaced the ends and installed them in a plane. They work but are very slow compared to the new ones. They work ok but I wouldn't want to try them in my faster planes.
That really sums it up pretty well. These days, $10 gets you a really nice, reliable, servo with pretty good speed and resolution. Compared to what was available years ago, it's down right amazing. I understand if you can't afford $30 on three new servos (use the old ones on throttle, no harm there), but if you can afford newer servos, it's worth it.