So, you've picked your race. You've picked your class. You're all excited for the character and empire you're set on playing when all of a sudden you're smacked with the choice of three spheres. "Oh, who even cares," you might think. "I'll just pick the ones that sound cool and have at it. I never end up researching that crap anyway" Right. And that's okay. Nothing wrong with that. But your spheres, or specializations, can actually have a pretty big impact on how you play the game.
That's why I'm writing this: To hopefully give an overview of what specializations are good at what, and when. Whether you're someone who's mostly ignored specializations (specs from here on out) up to this point, or if you're an experienced player looking for some thoughts on what works best, or if you're just someone who's looking for a reason to get angry about the way your favourite spell isn't valued highly enough - this is for you! I don't want this to be like a "Me talk you listen" thing, so definitely leave feedback so we can hash out a better understanding.
The word of the day is synergy: To me, the biggest factor in determining whether a specialization is a good choice or not is how well it interacts with what you're trying to do.
Air's an interesting beast. On the one hand, Suffocate is pretty underwhelming.spell - with only 15 , and the inability to damage a host of enemies, it's rare that you'll be dredging this one up. It does benefit from having a low casting cost, but you're still better off with just about anything else. However, it does have two potentially useful spells: 20px Seeker and Summon Zephyr Bird. Seeker is a powerful buff for ranged units, and Zephyr Birds are excellent for classes without a summonable, flying scout. This means Air Adept synergizes well with both the Archdruid and the Warlord - the Zephyr Bird fills the role of a flying scout, while Seeker is a great buff for ranged units, of which both classes have several. Finally, Domain of Winter is a pretty solid morale spell, as Arctic terrain is disliked or hated by quite a few races. It's particularly beneficial for Frostlings, who are almost guaranteed to start off in Arctic terrain.
In summary, Air Adept is a solid sphere pick offering a good scout, a powerful unit buff, and a good morale boost, and is a particularly good choice for Warlords, Archdruids, and Frostlings.
Whether to take a second Air sphere can be a tough choice. The fact that Arctic terrain will slow down your movement unless you're Frostling will mean that Arctic Empire might not be a desirable choice even if you've cast Domain of Winter on all your cities, and it has no higher-tier spells, finishing off with Summon Air Elemental - not a bad ability for those classes who lack late-game summons, but perhaps a bit underwhelming in compared to what other Masteries offer. Wind Ward can be incredibly powerful in the right circumstances, but is rather situational. If your army is mostly melee and the enemy's is mostly ranged (like, say, besieging a Dreadnought's city) then it's a lifesaver. However, if you rely on ranged damage as much or more than your enemy, it can make things even worse. As a spell it can work wonders, but it's far too dependent on the enemy's army composition to come all that highly recommended.
20px Haste, however, is a whole different ballgame. This is arguably one of the most powerful unit enchants in the game, effectively giving one or multiple units complete run of the battlefield. While overall it doesn't synergize particularly well with Air, the Rogue class might nonetheless consider taking Air Master just for this spell, as it can be particularly deadly in his hands.
In summary, Air Mastery is a bit more of a mixed bag than its predecessor, and may not always be worth the sphere pick. However, against range-heavy opponents like Elves or Dreadnoughts, or in the hands of Frostlings, it can be worthwhile.
Now this is a tough one to evaluate. Creation, perhaps more than any other sphere, has an emphasis on terraforming, with half its adept spells dedicated to this purpose. Cleanse the Land allows you to transform sections of your territory to Temperate terrain, while Domain of Life ensures that your cities will be happy about it.