From my experience martingale EAs work but they must be monitored. At one point I was running so many martingale EAs that I was getting about $400.00 to $900 per day. I didn't monitor it and a sharp move with no retracements killed me as margin kept doubling as each sell limit was hit.

However since then I have used them successfully by not starting too large for the account and knowing the behavior of the pair I am using. Average Daily Range and True Range are very important to compare on a daily basis.

In addition, I find that martingale EAs work best in a counter trend after the London session closes or when there are no major announcements.

Also when you look back at a pairs history you may not take into consideration what announcements were made or if Ben Bernanke, Tim Geitner or Barack Obama spoke to make the currency go crazy.

These announcement/speech days are considered abnormal and will more often than not affect the range and behavior of the pair you are trading. So you must decide to take smaller losses when these events occur rather than ride them out and sustain bigger losses due to the limitations of these EAs.

Most brokers will not go beyond 50 lots (live) so that is your limit regardless of how much money you have in your account. At that point you would have to trade manually to keep up but your profit potential may not be worth the risk.

There's my two cents.