Well, in retrospect (and literally, within a year of letting him go and getting Amare) it was already looking like this was a lateral move at best.
David Lee was a solid scorer/rebounder/playmaker - overall a really good all player. While his defense was suspect - lot of players of that era were not so good at it. Centers at the time were a premium.
If we could rewind this - the Knicks probably would have been better off giving their money to Lee than Amare. Both had their share of injury issues in the end, but if the Knicks were going to build the same case around them - getting Carmelo, Chandler, Jeffries... Lee would have been a better match alongside them than Amare - especially an Amare with no explosion. He would have gotten the defensive help around and could have complemented Carmelo with his better playmaking and no-nonsense approach.
In the annals of NBA history, I'd put David Lee, impactwise, along the lines of Carlos Boozer or Lamar Odom - very good player, borderline all-star, really solid NBA starter. Not as good overall as Amare (the Amare/Nash pairing was unbelievable) - his best years were when Steph Curry (and the GSW crew) were really young, and injuries had hobbled him to a reserve role. But, he did indeed play a solid role in the run that led that Warriors team to their 1st championship.