We used to go in September a LOT. Haven't been in September for a while, but I can't think it's changed that much. Weather is warm and humid (but I live in Florida, where it's HOTTER than SXM in the summer anyway!! <img src="http://www.traveltalkonline.com/forums/images/graemlins/Grin.gif" alt="" />). There is very little traffic. There should be no crowds at local beaches, except possibly on a Sunday, when many locals go to the beach. You do not need a reservation at pretty much any restaurant on island. Some restaurants will be closed, with usually more restaurants closed on the French side than Dutch. Some restaurants may not be closed entirely but only open limited days. So, don't get your heart set on a particular restaurant unless you have confirmed with them that they will be open when you go.

While it's unlikely, since September is the height of hurricane season, at least a tropical storm is POSSIBLE, so make sure that you buy trip insurance. If a storm is brewing, do take pro-active measures yourself and keep yourself informed and if necessary, stock up on some essentials like bottled water, candles, etc. Like I said you shouldn't need it, but just in case you might print out a list of hurricane supplies and preparedness tips at home and throw it in your suitcase. Can't hurt.

Enjoy your trip!