Bill,
Series has worked fine for about 50 flights. The down side of the parallel is that I have to run more wires hence more weight. The batteries cannot be easily charged due to differnt impediance of the batteries and consequently the glow plugs do not draw the same amout of current so you can end up with a glow plug poorly lit. I am using the series system with great success, and wouldn't heitate to recomend the set up to anyone else.
Sparky