We started with 'WP Super Cache'. It delivered pages very fast, unless it was the first time the page had been accessed this hour, and then it was slow. But every once in a while the page load time would get really slow. I recorded an 18 second page load using YSlow under Firefox. It happens about every 100 page views, something goes terribly wrong and some pages may never load at all!
So next we tried DB Cache reloaded. They claim to be really fast, by loading the database into memory so it can be read much faster. The load on the server went way down, but the average page load was 2 seconds, about the same time as it was without any caching program.
So now we are trying 'WP-Cache'. With it installed the average page load time is about 1 second. Of course we need to leave it on line for a while to see how Alexa and Google will respond to it.
This picture shows our page load time over several months. The sharp changes on the left 2/3 were recorded while using WP Super Cache. The gradual changes on the right were while using DB Cache.