Beware, long post:
I love this spell. If I get high on the threat meter I just switch to this. I also use it early on in fights to ensure I don't pull hate. Using it in a rotation is extremely mana efficient for solid DPS and for quick dangerous fights it's the safest spell to spam and the most DPS to take down a boss quick. The big downside obviously is that getting to the point where this is the best DPS spell in the game is torture on your mana pool. There are still several fights, especially in 5 mans and heroics (less so in raids... though I dont get as adventurous with casting in raids), where spamming this to take down a boss quick is arguably our best option. (Between Mind Controls on the second boss in Shadow Labs immediately comes to mind, spam this for maximum damage and so you dont have mana to hurt your mates when you get Mind Controlled, then Evocate/Gem your mana back when MC is over. Spamming this works well on a lot of caster bosses or bosses that hit hard but have low health too.)
For me the spell does around 1400-1500 on a regular cast (+700ish Arcane damage)with crits in the 2600-2700 range. At 3 debuffs that is at least 3000 in 3 seconds with the lowest threat and lowest resist rate through talents that you can get. The Tier 5 set bonus adds 20% damage and 20% mana per cast which is 2/3 of a permanent Arcane Power for this spell. That would be 1800ish per cast (slightly less than a regular fireball) for less mana at 0 debuffs and 40% less threat. At Full debuffs that would be... well besides no mana... almost twice Fireballs non-crit DPS (crits obviously favor fire).
Im a frosty at heart (though currently Arcane/Frost), so I cant offer much more in the way of a comparison to Fireball, but compared to Frostbolt I do find my current AB to be less "useful" at 1 cast (relative damage/utility), and comparable to FB in damage relative to utility at 2 casts (1 debuff.) I rotate it regardless for mana efficiency and lower threat depending on where I am on the threat meters. At 3+ casts it is quite a bit more damage than Frostbolt, though much more mana intensive making it more situational. My gear favors Frostbolt (Just under 900 frost damage, 700ish Arcane) so if I had equal +Arcane it may in fact be better than Frostbolt at 1 and 2 casts. I have thought about relevelling Tailoring to switch to Spellfire gear for Arcane Blast... but my hands are tied by the whole "Frosty at Heart" thing.
The beauty of a rotation with Frostbolt on longer fights is the versatility, and the option you have that after the first AB if you time it well you should be switching between 1 and 2 debuffs and not losing it completely unless its a mobile fight. AB > AB1 (1 debuff) > FB > FB > AB2 (debuff almost always wears off because of the timer then starts anew) > AB1 > FB > FB > AB2 > AB1. If you need more mana you can cast a third Frostbolt then go back to AB with no debuff. If you want more damage you can keep it between 1 and 2 debuffs or even go to 3 debuffs. Also, with this rotation snare will always be up if that is applicable to the situation. Sometimes with this rotation, that third AB will not reset the debuff, and Im wondering if it's because of server lag or my computers lag or whatever, but when that happens I usually let the entire debuff just reset to make sure I dont burn mana to quickly. I believe Fire mages who occasionally use AB use a different rotation.
Anyway sorry if any of that didn't make sense... things seem much clearer in my own mind than when I read it. That is all... discuss!