Two weeks of flash cards, every day. I have 712 cards and have done 1723 reps.
I don't like the passive nature of flash cards. After running through 90 cards in 20 minutes, even having understood everything on the cards, I don't actually feel any farther along. There isn't the same sense of progress one gets with grammar study ("finishing the grammar exercises in chapter 4") or an actual course ("finished another week of Assimil"). I'm just reading the sentences and clicking 'next'. Afterwards, I find myself thinking "What did I actually just learn?" I'm memorizing content, but not in a way I feel will help me produce it.
With Assimil, I can run through the dialog in my head and feel that I've learned something. I know the meanings, understood the grammar, listened to the recordings, and could produce those sentences if needed. The flash cards, not so much.
That's why I find myself drawn to active grammar study as well as my desire to produce. If I can't produce it, I haven't learned it. Passive recognition of the language is not sufficient. There is simply not enough thinking involved.
An example. Lets say my brain translates the words and produces the sequence: "later I had monkey a banana eating watched." My brain assembles (and yours probably did too) into the English sentence "Afterwards I watched the monkey eating a banana." But being able to recognize the words and understand the sentence doesn't put me any closer to being able to produce it. Flash cards are just rote memorization, but the second the prompt is gone (the sentence in the screen) so is the knowledge.
This is probably my math background talking again, but for me is does not logically follow that "successful flash card run == learning." There are simply too many variables in play, too many other things that could happen that could cause the English sentence to pop into my head when I look at the Dutch one. Maybe I'm just guessing the meaning from keywords in the sentence? "Something something banana something something something watched." Ah, that must be the sentence about watching the monkey eating the banana. There are others, but this is a proof by contradiction so one is sufficient.
That being said, I'm still going to keep doing the flash cards. If nothing else, it's a way to get me to study sentences from my various input sources and look back at them occasionally.
One thing I have never taken advantage of with flash cards before is the different types of cards. When I briefly did flash cards for French last january, I found it was too easy to make bad flash cards. So, I'm a bit cautious of trying other types. My cards at the moment are Dutch->English sentence cards.
Another area of potential flash-card improvement is moving towards single-language cards, something both AJATT and Antimoon suggest. I don't know if converting all of my cards would be an effective use of my time, or if it only pays off later once your entire deck is done. Katz says he switched when he was somewhere between 500-1000 cards, which is about where I am.
I similarly haven't decided about using monolingual dictionaries. There are arguments both ways but I think I fall on the "bilingual-dictionary" side, at least for the moment.