Despite its power, it's easy to see why this spell might not be first on everyone's list. As a 5th level spell, it's competing for attention against such favorites as Cloudkill, Telekinesis, and Teleport. Unlike many other D&D spells, it has no guarantee of working, in that the entities contacted may either A) not know the answer, or B) just plain lie about it. Lastly, there is a decent chance of the MU going bonkers if he tries for the "higher" planes.
Not quite as bad as this, but still...
There are several interesting things about this spell. First off, the planes are numbered "3rd-12th" in the spell description, with no real mention about what the sequence refers to. I assume the 1st & 2nd are the material and the ethereal planes, respectively. DMs are expected to assign the others as they see fit, I expect. Personally, I approve. I'd hate to shoehorn a cosmology into my game that doesn't fit for the sake of one spell.
Secondly, the idea that the wizard can push this spell as far as he wants, choosing the plane he tries to reach. Want to play it safe and just ask on the 3rd plane? Go ahead. You won't get to ask as many questions, but you'll probably keep your marbles, and whatever picks up the phone is less likely to lie. They are also less likely to know the answer to begin with, however. If you kick it up a notch, the risk/reward goes up too. I love this kind of thing, where the player has to weigh their choices.
I also love that the DM can feed the players false info. I'm not trying to screw over the PCs, but it's sort of what you'd expect from some extraplanar being that got the equivalent of an annoying phone survey call right at dinner. While the lying could take the form of the entity claiming ignorance, the opportunity for mischievous misinformation here is more than I could resist.
Last, but not least, almost like it's straight out of Lovecraft, is the insanity factor. The idea that the caster could just go gaga with NO save is pretty impressive. Granted, the insanity is temporary, but the wording also states he is "out of the campaign." In other words, this isn't a case of slight anxiety, this fella is wigged out and no use at all on an adventure.
It sort of makes you wonder what the wizard wants to know THAT badly. The answer is usually 'nothing' so the players never use the spell. While it is tempting to drop a scroll or spellbook containing CoP into the loot to see what the party does with it, I think this is the kind of spell that would need to be used multiple times before all the different wrinkles it presents could be explored.