Toughness has undergone several changes in our games.
(1) Toughness upped the HD by one step, to a maximum of d12 (since nobody is tougher than a barbarian!).
(2) The splatbooks came out, and we used the various Monster's Toughness feats.
(3) That was unsatisfying, so we went back to (1), until someone pointed out to me the obvious ... it made no sense that a barbarian couldn't gain any benefit from Toughness. So I changed it to +1 hit point per level, retroactive. This was practically the same, except for the slightly reduced benefit at 1st level.
(4) Complete Warrior was released, basically codifying the standard "fix" for Toughness, but with a prereq. So we changed it very slightly, such that if one takes Toughness, upon reaching the prereq, it auto-morphs into Improved Toughness. (Yes, this means that some weaker classes get slightly more utility at low levels from Toughness than Good-Fort classes get. THey need it.)
Through all these iterations, it has never become a Must Have feat. From all four of our current games, only one PC has Toughness/Improved Toughness.