I'm excited to see so many people have positive experiences with this deck! I personally have been getting equal losses to wins, so it's not 100% viable as-is. That said, I'm not that great of a Hearthstone player. I make lots of mistakes in games. Your milage may vary based on your skill and luck.
As per user ra1ndrop's suggestion, I've been trying replacing Spellbender and Pyroblast for Vaporize and Cablist's Tome. I'm still having about the same 50:50 success rate, but this may fit your playstyle better than the default deck.
I've noticed some players with quest cards have caught on to the Kabal Lackey + Counterspell on turn 1. Normally most people only have The Coin as a counter to this "combo", but Rogues have Counterfeit Coin, Backstab, Preparation, and possibly Shadowstep as potential counters. In any case, them losing a card is always better than nothing.
Again, if you find changes to this deck that work for you, let me know and I'll try to play them!
I'm personally having mixed success with this deck. Usually 6 wins to 4 losses per 10 games played. Still very draw-dependent, so luck does play into it. I'm going to play around with different cards and post back with what looks better. You will want to replace cards as you see fit for your playstyle.
To be honest, I'm not that great of a Hearthstone player or deck builder (this deck is largely based on the deck mentioned below, with small changes to make it standard), but I have a lot of fun with this deck. This deck is built around being fun to play instead of strictly a competitive deck.
Please note the new Mulligan guide since it includes a fun quest card killer.
Modified (Rank 20-5) Aggro-Midrange Secret Mage deck. Card replacements have been made to have more useful secrets and more consistent secret pulling (praise Arcanologist!). I've gone from 17 to 13 with this deck over the past few days, so I'm not sure if you can really 20 to 5 like the original.
Replacing Wild Cards from the Original Aggro-Midrange Secret Mage Deck
If you play first: Kabal Lackey and Counterspell are quest killers. If you counter someone's quest card, they'll often times quit (which is pretty funny). Additionally, don't keep Mana Bind since you'll likely end up with a quest card that's useless to this deck. That said, quest cards may fall out of favor for most classes over the next few weeks, so adjust as needed.
I'm not very good at card choices in my decks, but luckily this deck is pretty flexible for what you can swap out. Be sure to comment what you think is good and what you think is bad and I'll try it the suggested changes.
Generally speaking, the core of this deck is Babbling Book, Kabal Lackey, Arcanologist, Medivh's Valet, Kirin Tor Mage, Ethereal Arcanist, Kabal Crystal Runner and a healthy mix of secrets. If you swap out a secret, you should really replace it with another secret.
In the original deck, Potion of Polymorph was included, but I've taken it out because of how it interacts with Mirror Entity. It's no fun to have your opponent play a cheap card, then blow two secrets on copying a 1/1 sheep. This is probably the only secret combination that I dislike enough to suggest not using unless you can afford to leave secrets in your hand.
Pyroblast may be swapped out with another card to suit your tastes. You'll often burn through cards in your hand quickly, so I find it helpful as a final blow. Pyroblast has been swapped out for Cabalist's Tome in the latest deck build.