After reading this post on Confessions of a Language Addict, I've started thinking a bit more seriously about "the commitment problem" with spaced repetition which I recently wrote about. To summarize the problem: Spaced repetition works great if you are committed to quizzing your flashcards everyday. But inevitably, you will have to stop for some period time. When you come back, the number of expired cards could overwhelm you.
I was using Memorati™ religiously while I was reading the first Harry Potter book. It was fantastic. After finishing the book, I stopped doing flash cards for about 1 week. I needed a break. I have over 2.500 cards and I was on a very strict regimen. Several weeks have gone by and I have still been unable to catch up.