T56/Alison 501 series are not very efficient but in terms of unbustable instant power they are hard to beat. However while they are many still in service I believe it has been over 15 years since any new ones were built.
The poor Aeroproducts 606A propeller has IMO gotten an undeserved bad reputation. It takes a bit of work to understand it but I think it is an absolutely brilliant design.
As for the longest in production turbine engine I would say it would be the PT6 series turboprop, first flown in 1961 and still being built in the same basic configuration (for the lower horse power single wheel versions) today.
Last edited by Big Pistons Forever; 15th May 2012 at 23:20.