At a glance the cards seem like a way for characters to largely gain buffs of varying degrees of power, at no cost to the character(the player gets to foot the bill). Many bonuses are highly situational, while others require a die roll to see if they work in your favor. I want to make it clear that I haven't gotten a chance to Fortune Cards much: I was given some free packs from my local store in order to see what my group thought of them, and got to see them in action when I ran the first session of March of the Phantom Brigade (last night).
The greatest issue during our Monday night game was that we kept forgetting to use them, or discard them if we didn't like what we had. Only one player burned through them with any frequency, while the other players managed to use one or two. In the end we quit using them halfway through the session: while Liz and Beth seemed to get a kick out of them, Randy didn't care for them at all. It wasn't that they overpowered the characters, or even really provided a noticeable increase in power, but hat they added another layer of complexity that they weren't used to.
Personally, the hardest part was getting a card that was relevant. One card let me knock someone prone if I fell prone, another let me shift my Speed, and another would reduce falling damage. The only problem? Not a lot of monsters seem to have powers with prone kickers, it's rare that players seem to be climbing about high enough that fall damage is a concern, and as a defender I really never wanted to shift insanely far (especially when I could fall down if I rolled a 9 or less).
During Encounters, everyone except one player used them, and that's because he still had Rewards Cards from past Encounter seasons. During a skirmish against a quartet of blood-thirsty stirges, only two players really used them, but then two out of three ain't bad (and the player that didn't just kept forgetting to cycle hers out). Again, I didn't notice an increase in power. The encounter was rebalanced for a party of four, and at the session's end most of them were down 2-4 healing surges and an Action Point or two. Given that they can't take an extended rest whenever they want, this isn't a good start.
One player suggested doling out one per encounter as opposed to one per round. I think that this would be better for constructed decks, as otherwise the benefits can be quite forgettable. Another thing is that I like the Renown Point system in Encounters, and could see giving out cards to players that do sufficiently badass things. Kind of like Achievements, but with a tangible award. Finally, they could be an incentive award in lieu of handing out bonus XP. If you're going to do one of the latter, I'd recommend giving out a card that is actually useful, as I'd hate to do a lot of cool shit only to be award with the ability to reduce falling damage.
If you liked Rewards Cards, have a smaller group that usual, like the mutations from Gamma World, and/or want to add randomness to combat, then you'll like these (with or without controlling their distribution). If your table has one or more competitive assholes with lots of disposable income, or don't like having players gaining randomized benefits at no penalty, then you won't like these. My recommendation is to attend a few Encounters sessions and use someone else's deck and see for yourself. I don't think these were as bad as a I feared, but they're definitely not for every group.