As someone who did a lot of damage in Zendikar drafts with Archive Trap, I know the value of taking away 13 cards away from your opponent. In Limited, you're often playing against 40 card decks. But what made Archive Trap even better is that if your opponent searched their library at all, you could cast Archive Trap for free. I actually ended up hitting all of my opponent's lands of a particular color one time, and they failed to search with their fetchland (I believe it was Marsh Flats seeking out a Swamp or a Plains). Startled Awake is pretty much always going to cost you 4 mana, but by the time you cast this in draft, it's going to be well worth spending your turn 4.
What makes Startled Awake so good is that you can pay 5 mana (3 and 2 Blue) to bring it back as a creature. What it becomes, Persistent Nightmare, is only a 1/1. But because it has Skulk, as long as your opponent doesn't use removal on it, many times you can actually get it to connect with your opponent directly. Suddenly, you get Startled Awake back in your hand and you're able to mill another 13 cards! Also, in the late game, if you happened to discard it for whatever reason, you'll often be able to get a second chance at casting it. If it's a really grindy game, you may even get a third chance! It's actually a very fun card to play with.
Perhaps it's nowhere as efficient as Archive Trap was, and that Trap is still played in Modern on occasion! But Startled Awake can take over a game on its own, which is why it's a mythic rare. Imagine if this were a rare and you opened two of these in a Limited environment. You're going to win a lot of games.
As of early October 2016, Startled Awake is a $1.50 mythic rare. You can find it as cheaply as $1 if you know where to look. The cool thing is, many buylists pay $1 or more to get it in stock. Why? Because mill is a huge Kitchen Table Magic theme and a lot of mill players want 4 of these at a time. For players on a budget, Archive Trap is sometimes actually not as good as Startled Awake. A lot of casual players don't use fetchlands or many tutors, so you'd be paying 5 mana almost all of the time. Spending $6 over $36 is also a lot more palatable to people who just want to waste away a Saturday afternoon at the kitchen table. And Startled Awake is a lot more fun to play. Plus, many mill players will just play both in their decks if they happen to have them already.
(To be fair I know a lot of casual players who have 4 Demonic Tutors and Vampiric Tutors and full sets of all 10 fetchlands. Heck, I know a casual player with all of the Power Nine, including a near-mint condition Black Lotus! But not everyone does.)
What I like best about Startled Awake is that it is one of the most liquid cards you can find. It's so easy to pick these up at $1 or $1.50 and trade them away for $2. These are the sorts of cards that I like to collect when they hit rock bottom price wise because they really have nowhere to go but up. This card is only passable in Commander, so it's all casual demand here. And since you really do want four copies, they are usually going to disappear more than one at a time. Sure, Shadows Over Innistrad is an extremely popular set and there are a billion copies of these around. But eventually, these sorts of mythic rares become $3, $4, $5, or more. Even if it takes years to get there, I know that I really can't lose money on these unless everyone in the world stops playing Magic the Gathering.
So the next time you see this card, and you need to find something in a binder to make up a difference in a trade, Startled Awake is a great target. Someone need a dollar and change to buy a soda or snack at the Local Game Store? Startled Awake is great compensation (yeah, people do this!) It's a flip card, so unlike many casual favorites, it's unlikely to get reprinted any time soon, if ever. I love this card. It's fun to play and at kitchen tables, playing four in your deck is actually pretty feasible. I'm not even speculating on this card. I just really like it and want it to appreciate like all mill-happy cards like it have. It's free money.