So are people actually expected to pay 15 mana to get a beastie into play? What about time-tested concepts such as mana curve? What am I supposed to do in the early turns? Just play land and say go?
I can see use for them as a stalemate-breaker. If by turn15-20, victory is still far from certain, forcing a legend into play can easily turn the tide in your favour. But to make one the cornerstone of your deck strategy??? Most of the time, they seem like they will just sit impotently in your deck or hand.
Not so much...
Here's an example that happened to me in the prerelease.
Turn 1: Land, go (it was 5 AM and I messed up, I should have played my Joraga Treespeaker, but forgot)
Turn 2: Land, Joraga Treespeaker
Turn 3: Land, Level up Treespeaker (it can not tap for GG)
Turn 4: Land, Ondu Giant (4-mana 2/4 that searches for a land and puts it into play tapped)
Turn 5: Land. Tap my 6 lands plus treespeaker for 8 mana, Ulamog's Crusher.
Turn 6: Land. 9 mana for a Gelatinous Genesis to put 4 4/4 oozes into play
I also had several games where I got out a turn 4 Sphinx of Magosi (3UUU, 6/6, flying, 2U: Draw a card and put a +1/+1 counter on it).
I don't think I ever got 15 mana during the 6 rounds of the prerelease (around 15 games total), but I was also really starved for some of the better mana accelerators. In particular, I had only a single playable spell that produced Eldrazi Spawn.
Eldrazi Spawn are 0/1 tokens which are produced by a number of different spells (a couple just produce them, a number of creatures make them when the creature comes into play, there's an enchantment that creates them every turn, and then things like direct damage + a spawn, search for a land and get a spawn, etc). The spawn can be sacrificed for 1 mana, allowing you to store up mana.
So yes, a 15 mana megabomb like Emrakul is definitely playable in limited if you have the cards to support it. It is also definitely playable in casual constructed, though only time will tell how effective the eldrazi are in more competitive constructed environments.
It should also be noted that Emrakul (the 15 mana eldrazi) is a mythic rare, and thus very, very uncommon. The highest-cost rare is 12 mana (It That Betrays), uncommon is 11 (Pathrazer of Ulamog), and "castable" common is 8 (there's a common eldrazi that costs 9, but it's actually one of the weaker eldrazi; it has the ability that rather than paying its cost, you can sacrifice 4 eldrazi spawn; if you can't reliably do that, it's really not playable because it's just too weak to justify the cost)