I've had my Oregon Aero seat cushion for 9 months now and have flown about 500 hours in it in both a S-76 and a King Air 200. I find it really makes a difference when flying more than 3+ hours a day, especially in the S-76. I'd say it gives me at least another 2-3 hours of comfort that I otherwise wouldn't have had.
I'd highly recommend that you spend the extra money and get the optional lumbar support piece. It is detachable and make a big difference in supporting my lower back. Additionally, you need to consider getting the "Aerobatic/Law Enforcement" model that has the cut-out for a five point safety harness and/or cyclic. I have the medium thickness model which works well for me (6' tall). If you are taller than say, 5'10", the thicker model may give you clearance problems if you fly in the 206/407 series.
At least for me, while on the expensive side, it was a great investment if you fly for a living. Hope this helps.