About eight years ago I had one screw pile job for a large manufacturing building - on old fill that was too deep to over-excavate economically.
The performance of the building is fine with the piles. The problem we ran into was the lack of contractors who were familiar with and had the special installation equipment for this type of pile system. It does require specialized, non-standard equipment. We were at the mercy of the contractor's schedule as he was the only one for MILES around who could do the job, and he knew it. This drove the price up, and the extended the building erection timeframe. There were hard feelings between the pile contractor and the owner before the end of the job.
Just something to think about. Personally, I will not use screw pile again unless there are muiltiple contractors who are available to bid. Goodd luck.
Mike McCann
MMC Engineering