Temporal Trespass may appear to be a bit pricey at 8UUU (8 colorless, 3 Blue), which is a total investment of eleven mana. But most extra turn cards that have been printed have been costed similarly. The major advantage of this card is the fact that it includes the Delve mechanic. For each card you exile from your graveyard, you can reduce the cost of casting Temporal Trepass by one colorless mana. That's right: If you have 8 cards in your graveyard, you can take an extra turn for only 3 Blue Mana (UUU). This is an extremely attainable thing to do, especially in blue-based decks that use a lot of instants and sorceries as part of their control schemes. This card becomes extremely valuable late-game, when an extra turn can simply finish the game.
Exiling itself after use does prevent Temporal Trespass from being too horribly broken, but this doesn't mean that its effect can't be copied while it's still on the stack. Duplication effects can still take good advantage of this card. I see this being an extremely playable card in a wide variety of formats. Commander probably didn't need another extra turn giving card, but Constructed formats should have a field day building a deck that utilizes this card to its maximum potential.