This is all IMO - just wanted to get that off first
Since 3e first came out, I've spent half of that time playing a cleric (whether in a face to face or online game) mainly because I was usually the last to join the group - and you guessed it, that was the last "slot" that needed to be filled.
...and no, I'm not bitter about that because I've come to enjoy playing clerics
Now, gathering what I've learned playing a cleric in 3e in various groups (not to mention prior cleric gaming in 1e and 2e), I've generally experienced this:
Besides the usual toiling me and the wizard usually have in deciding what spells to memorize, as well as deciding the appropriate time to use such spells, the difference is with the clerics "spontaneous curing" ability.
Lets say I love spontaneous curing because it opens up a lot of options, but it's also a double edged sword. In 1e, all I had to do was say "I memorize Cure Light Wounds for all my first level spells, Cure Moderate Wounds for...etc". In 3e, I've actually had to put some more thought into spell choice since for once, I can actually pick up some "non-curing useful spells) for once. A great day indeed!
However, in turn, that affects the "when should I use a spell decision". This is where it depends on the DM and game style: some games have plenty of resting so that the spellcasters can burn off spells, while others have less resting so that spell casting has to be somewhat on the conservative nature, so hard choices have to be made.
More Resting = cleric can use spells more for buffing, combat, non-healing, etc.
Less Resting = cleric (or spellcasters in general) must maintain a balance for a possible next encounter, with a lot of "what if" decision making
Then that brings in the availability of "other healing" whether it's because of other PCs (another cleric, druid, bard, paladin, etc) or availability of potions, wands, etc. that helps with healing.
More "other healing" = takes the burden off clerics to maintain a certain level of healing spells available
Less "other healing" = party depends on the cleric more for their primary healing.
Hence, that finally leads to my: "Is the cleric overpowered?" IMO, and this is going to be a cheap answer, but it depends. In a "less resting, less other healing" party, the cleric will almost very likely be like the 1e cleric where all of their spells will likely be used for healing. In a "more other healing" party, less of a burden is placed on the cleric, and they can do some nasty stuff (I love Divine Power, but have only managed to use it once in 3 years

).
At the moment, I am in a game that's "less resting, more other healing", so that I generally keep enough healing for emergencies during combat, but there's an abundant of healing post-battle.
I have to say it's a bit of a transition because most of the games I've been in prior usually were "less other healing" and that made me much more conservative in spell casting non-healing spells. Someone recently had to remind me that I can use Cure spells as an offensive attack against undead, since it's been so ingrained in me that I might need that Cure for "later".
