Agree on the usefulness. I don't know if that's universally true of sargent (the harder grinding), I've had two, but never got along with either quite as well as the stanley.
As to the stanley planes, there has been a noticeable difference in some of the irons. Some of the old laminated irons were almost as hard as modern irons. The irons from the '70s are almost down to saw temper. The irons in between there are just lovely, especially on an oilstone.
Any of them are lifetime tools aside from possibly needing to replace an iron if you could manage to work your way through one - something no amateur will do with proper grinding and honing.
The English have it even better. Their record planes are about 20 pounds over there pretty much all the time, and they are the equal of the stanley planes but with laminated irons through a much later time. The SS planes with a two piece cap are heavier-made. The wooden planes over there are absurd. I bought a try plane from over there for the equivalent of about $35 (it cost another $35 to ship). It had a brand new ward double iron in it. I couldn't make a better plane (it did need a little bit of refitting, but after that) with $100 in materials. The response I heard from their folks on the UK forum was essentially "i'd never pay $35 for a plane. That's too much".
Pretty sure that in the days those planes were made, they'd have cost a journeyman a day's wage - until later in time when wooden planes became really cheaply made and cost half as much new as a stanley.