You would think it should blow up with 3 simultaneous themes but it works fine. Also work fine with Dave's WordPress Live Search (ajax-powered), Akjax edit comments etc.
It didn't work with Hypercache for me and Super cache made naff-all difference. My Yslow is now 89. I wouls be better if my theme didn't have so many images.
In short, this works and is worth trying. Backup first as your mileage may vary.
I also found their support was good via the Google code pages.