MT - thanks, unfortunately I find it difficult to trade during the day (work!) so the ability to leave automatic trades is very tempting. The main pitfall appears to be a sudden permanent move which might land you in a position you don't want, although presumably you could setup an automatic follow on order to limit your losses.
I have a couple of shares in mind. When I used to hold shares, I had a trailing stop loss triggered on a quiet day and ended buying back in (which fortunately turned out to be the right decision) however, someone made off with a few hundred quid at my expense. Another hard won lesson.