Good? Very good? Sure. Overpowered? Not by a long shot.
It comes in the versatility of the Four Roles (healer, blaster, tank, and sneak). A well-tricked-out cleric can supply two of those four pretty easily (tank, healer), and they are one of the only classes that can do healing. The bard can cover Sneak and Healer pretty well, but without armor, they can't be much of a tank. The druid can cover three roles pretty okay: healer, blaster, and tank; and, arguably, they can make the fourth one in certain situations, too.
Now, the areas that clerics don't do that well in (blaster and sneak) they can still be made to do somewhat decent in. A cleric with the Fire domain, for instance, becomes a potent blaster -- not as much as a wizard, but then, that cleric can still heal and tank, while the wizard can't do either of those. A cleric with the Trickery domain can become a decent sneak, using illusions and Charisma to compensate for their bulky armor. They won't be as good as the Rogue, but the rogue can't tank, can't heal, and can't blast, either.
This leads to the perception of clerics being overpowered -- they can do a little bit of everything (and usually, more than the bard can). Where they are limited is that the only thing they do better than anyone else is heal.
You can be a cleric-tank, but why bother when you can have a fighter who will be better with less effort? You can have a cleric-blaster, but if you've got a sorcerer, who needs it? You can have a cleric-sneak, but the rogue will outshine you in every situation. You can have a cleric-healer, but there's two other classes who can use wands of wound-curing too, so that's hardly a monopoly.
Which, ultimately, means clerics are great at support. They can help any member of the party in any situation, though they will only outshine other party members in healing. They can run to the front with the fighter, they can hang in the back with the wizard, they can sneak off to back up the rogue. This makes them one of the most versatile classes in the PHB, perhaps *the* most versatile. And frequently, versatility gets confused with power (cf: every argument on the Mystic Theurge). They aren't the same thing, however.