Stephen
Yep a big con is cost !!
Pros are, low noise level, low vibration, more compact size, Designed to run at full load 24/7 , more air for the same given motor power hence lower running costs,,
They are a few makes of screw compressor about, and cheaper, but went for the HPC as being recommended as the best, so much so, have forgot what i paid for it,, remember it was not cheap,,IIRC about £1k more that a piston machine, but even with hindsight, would buy the same again