I think think the Sandisk / Condusive ReadyCache product is good. Especially since the release of the 1.3.2 software update, there have been relatively few bug reports.
17 GB fill of the default 29.xx GB partition is the lowest reported fill where delays occured, that was AlleyViper. All the other reports I found started at just over 23 GB, there were several in the 23.xx range, and then up from there. It bears noting that the cache is still working at 100% fill for almost everyone, the task is mostly just trying to improve performance at this point.
