I park 100+ domains. From January 1 to May 31, 2009, I earned $114.56 with a 2% CTR at my domain parking service, SEDO - even after working for several months to optimize returns with them.
Prior to SEDO, I tried other domain parking services. Other than a brief window with one service's high returns that later faded, I have found SEDO's returns to be comparable.
In June, I began redirecting all my parked domains to the Amazon home page or Amazon Kindle page (Kindle pays a 10% referral fee) via Amazon Associates. For an example, go to http://gpsbeacons.com or http://commontunes.org
From June 24th to August 24th, 2009 (just two months compared to the five months at SEDO above), my parked domains generated 209,424 clicks, referred sales for $2,184.27 in merchandise at Amazon and earned $156.51, approximately a 3.4x improvement over SEDO.
In other words, I earned more in two months with a simple redirect to Amazon Associates than I earned in five months of keyword optimization at SEDO.
Keep in mind, I have not done any keyword or domain optimization to target specific Amazon categories, products or sections.
It makes me wonder what value the domain parking industry is able to bring customers at this point.