I've played with Tradestation, Multicharts, Ninjatrader and Openquant. They are all great in their own right but I had one experience that was an eye opener. I tested a system by looking at charts and noting the trades, I tested the same system with Ninjatrader's backtester and also with Openquant's backtester and I got three entirely different results. I never could account for that but it changed the way I do testing. I do some manual testing, then I write an indicator [not a strategy] that performs the same work I did manually, then I proceed to a strategy that I can backtest [currently that is done in Openquant] and I also run it forward in realtime against a simulated account and I compare all the results. Anything less than that is not satisfactory to me considering that the backtesting industry is sort of in a stage of infancy and you can't really predict slippage and missed trades in a backtest...