Weird. I find 2-back pretty easy, but 3-back difficult (I normally get 50-80% accuracy) and 4-back quite tough (20-50% accuracy). I wonder what a typical dual n-back level is.

Edit - OK, pulled up some actual data for anyone else who's curious. The 35 University of Bern students trained in the 2008 study by Jaeggi et al. had a mean dual n-back level very close to 3. After 8-19 days of training that increased to 4-5. The 25 National Taiwan Normal University students who trained on dual n-back for this 2009 Studer et al. poster went from a mean n-back level of 2.0 (with a standard deviation ~1.1) to ~4.6 (standard deviation ~2.3) with 20 days training.

I'm surprised by the discrepancy in scores. I can also do more than 9-back and personally know people who are considerable smarter than I am (e.g., Nick Bostrom and Robin Hanson). I suspect that the n-back game does not very strongly correlate with IQ, or else that scores in this game can be dramatically boosted by the use of certain subtle mnemonic and visualization strategies, which even intelligent people may fail to adopt.