The notification of your question landed in my spambox, so sorry for the extreme delay. While SimPyLC has been constructed with care, it has some limitations:
Since it's just running as an interpreted application in a garbage collected language (Python) there's no guarantee that it will always react fast enough. Especially on a general purpose computer other processes may cause the sweeptime to become longer (although I've never whitnessed that). It also hasn't undergone the kind of rigorous quality checks that an industrial strenght PLC has to undergo.
Still it is my experience a very powerful tool to simulate controls in advance. I've used simulations like this with great success for large projects, like one of the very first fully automated container plants here in Rotterdam.