I don't even know if this is a controversial statement at this point, but I think Delver of Secrets is not just good, but both the best creature and the best card in Standard.
In a relatively short time, Delver of Secrets has grown into one of the most influential creatures—influential Magic: The Gathering cards—in recent memory (maybe history).
Here is my blue white Delver of Secrets Deck:
* Main Board (60)
Colorless (25)
1 Plains
4 Seachrome Coast
1 Batterskull
2 Sword of War and Peace
3 Moorland Haunt
1 Runechanter's Pike
9 Island
4 Glacial Fortress
Black (1)
1 Dismember
Blue (28)
4 Mana Leak
4 Ponder
4 Delver of Secrets / Insectile Aberration
2 Invisible Stalker
4 Snapcaster Mage
4 Gitaxian Probe
2 Thought Scour
4 Vapor Snag
Multi-Color (4)
4 Geist of Saint Traft
Red (2)
2 Gut Shot
* Side Board (15)
Blue (6)
1 Jace, Memory Adept
2 Phantasmal Image
1 Negate
2 Dissipate
Colorless (1)
1 Batterskull
Green (2)
2 Corrosive Gale
White (6)
1 Divine Offering
2 Timely Reinforcements
2 Celestial Purge
1 Revoke Existence
I have used this deck on Standard tournaments and have never lost. Although this is before the release of Avacyn Restored which can change the tide for Standard format this year.
What did good players think of Delver of Secrets?
This is something that I was interested in and a question I asked myself when I first realized how influential Delver of Secrets was becoming.
... did writers (in their various "Innistrad review"-type articles on the websites various) realize at the outset how good Delver might become?
Quiet Speculation:
"This is extremely marginal but if it's possible to set this up as an early 3/2 flier consistently, it may see some play."
ChannelFireball.com:
Luis Scott-Vargas rated Delver of Secrets a "2" ("niche card"), focusing on the card's awesome flavor (and I think we can all agree there is some nice Jeff Goldblum going on on the card), although he did speculate that it might be "sweet" and possibly where.
TCGPlayer.com:
Conley Woods was the most optimistic about the Insectile Aberration-to-be, rating the one-drop a"3" (his "backbone of Standard" level, comparable to Makeshift Mannequin or Mind Spring)... but acknowledged that some might see this as a stretch. While Delver of Secrets didn't make Conley's Top 10 list for all of Innistrad, it did make his "Top 3" mini-list for blue, saying it was one of the cards he was most interested in working on.
My goal isn't to call anyone out on this front, but more to illustrate that even the experts can miss (or at least underrate) some of the most powerful cards.
I think that in this case, that came from not realizing what a very little amount of tuning (playing lots of spells and a four-pack of Ponders) could do for the Delver.
So what makes the Delver so awesome?
Essentially, it redefines a lot of what it means to be a blue creature. Delver of Secrets is aggressive. It is a one-drop... you can play it before the stage in the game where you "need" to keep your lands open for Mana Leak or whatever. You can put other decks on notice (while threatening to take over with the aforementioned Mana Leak). And what was really underrated initially: Delver of Secrets hits hard!
Have you ever seen one of those draws where one player goes turn-one Delver of Secrets, turn-two two Delvers? I mean 9 damage coming in on turn three, over the top of potential blockers, is impressive almost unconditionally... but from a blue deck? Ka-pow!
Factor in all that stuff about holding back the old Mana Leaks, and you have something pretty special.
Did you ever come to the realization, looking at an Extended blue deck splash just for Tarmogoyf, that the age of Keiga, the Tide Star in tapout strategies might be over? Delver of Secrets is that, over and over again ten times.
Why is Gut shot on his deck?
(And how do I even cast that with my blue and white lands?)
Gut Shot is amazing in the Delver deck!
You can Gut Shot a small creature, tap out for Snapcaster Mage, and Gut Shot again.
Gut Shot is one of the best cards against the viable non-Delver of Secrets decks. Red-green variants from beatdown to Naya Ramp rely on cards like Birds of Paradise to get the jump on other decks. Gut Shot—being "zero" mana (especially in a deck that can't cast it legitimately)—lets a white-blue deck get a similar mana advantage, interactively, to a red-green deck advancing actively with an actual accelerating one-drop.
Other links you may also want to check out:
Vexing Delver Deck
Gideon Jura's Soldier Deck