In comparison to other max players, it is hard to say he is worth the money.
Again we have a problem with the terms we're using here.
What other "max players"? Do you mean players making $25-30 million a year? Or do you mean players beyond their rookie contract being paid at the maximum annual value allowed for their level of experience?
There are a lot of players in the league making over $20 million per year who received the highest possible annual value at the time they signed their contract, or close to it. In other words, they're making max based on what the CBA allowed. Horford is more valuable than a lot of those guys.
Horford is not as valuable as LeBron or Curry, sure. He's not better than Dame Lillard, or Chris Paul, or James Harden, or Kevin Durant.
But those aren't the only guys making $25-30 million.
Is he more valuable than Paul Millsap? ($30.8 million) ... Yes.
Is he more valuable than Blake Griffin? ($29.5 million) ... Yes.
Is he more valuable than Mike Conley? ($28.5 million) ... Yes, I say, even w/out injuries.
Is he more valuable than Carmelo Anthony? ($27.5 million) ... Yes.
Is he more valuable than Andrew Wiggins? ($25.3 million) ... Yes.
Is he more valuable than Jrue Holiday? ($24.9 million) ... Arguable, but I say yes.
Is he more valuable than Otto Porter? ($24.8 million) ... Yes.
Is he more valuable than C.J. McCollum? ($24 million) ... Yes.
Is he more valuable than Hassan Whiteside? ($23.8 million) ... Yes.
Is he more valuable than Bradley Beal? ($23.8 million) ... Arguable, but I think I'd say yes.
Is he more valuable than Andre Drummond? ($23.8 million) ... I think I'd rather have Horford.
Is he more valuable than Dwight Howard? ($23.5 million) ... Yes.
Is he more valuable than Chandler Parsons? ($23 million) ... Yes.
Is he more valuable than Harrison Barnes? ($23 million) ... Yes.
Now, about many of the above players you could say, "Well those are bad contracts." That's true. You could also argue "Many of those players are paid based upon age and potential." That's true as well.
But that's also just how the free agent market works in the NBA. Age, years of experience, how much a player is allowed to be paid under the CBA, what other players are available in that free agent period, etc are all taken into account.
Horford is worth as much or more than many other players who make a similar annual salary in the NBA. You may not feel that most of the above players should have gotten close to max money, but the reality of the league is that any above average free agent is going to get a big deal, and if you're a team with cap space that wants to sign a good player, you're probably going to have to offer the maximum annual value allowed under the CBA.
If your argument is opportunity cost, i.e. the Celts could have signed somebody else who is more valuable, or they could have signed multiple guys for the same total amount of money, I just fundamentally disagree with you.
But the bottom line is that somebody was going to offer Al Horford a max deal and his performance over the course of his deal so far is absolutely commensurate with that salary, i.e. he is a top player on a quality team, provides a lot of leadership and intangibles on and off the court, and has helped to attract other good players to join the team.