I have been using a similar concept for some time. My deck is similar to what
barsoomcore posted on these forums years ago. I cannot find bar's old post, but below is an example of what he had.
http://www.scratchfactory.com/Resources/SwashCards.pdf
I only allow 1 card a game session which can be played at any time. Every group I have ever introduced them in loves them. Once introduced I have never been able to pull them back out of the game. I pulled them out when I first move to 4e and the players were unanimously asking for them to be added back in.
As a GM I love them as it throws plot twist my way which makes me think on my feet. It's a challenge I get a lot of satisfaction from. The players love them as they love to see the DM squirm and get a kick out of the unplanned twist this brings all of us. This forum talks about Rat Bastard DMs? I have Rat Bastard Players and I love it!
Are some of them "combat win" cards? Sure. I have one in my deck that says reinforcements show up. This card shows up at least once every four sessions (Murphy's Law and all) and often has turned a planned tough fight into a cake walk. The fun was figuring out what that meant when the party was miles from civilization in a swamp or far into a lost shrine of undead was tough for me to figure out. In those two cases I had an alligator flank the bad guys in the swamp and a spirit that was looking for redemption help in the undead lair. Both experiences are events that have been talked about fondly for years.
Can I see some groups not likely the extra boost? Sure. I get it. To each his own. Kudos for WotC for coming up with a product that for some groups will be a lot of fun. I'll stick to my own cards as my group have become attached to them. That said, I hope they make gobs of cash at this which they can use to invest in products I do want.
Good gaming to you all.
Vaslov