We're testing with both. Somewhat hindered by the fact that WT often dies trying to import our data, or crashes, or loses data in weird ways. :( I believe WT will eventually get there, but rocks has been unfathomably more solid for us so far.
WT has been pretty decent for us (only TB level amount of data, so probably not as much as you folks are dealing with). The thing it has issues with is occasionally using the wrong indexes and caching them, and even putting hints in does not solve the issue in all cases. It started happening on WT, but it seems to be mongo's issue and not the storage engine.
We have a lot of edge cases in our data set and usage model, no question. Glad it's working well for you. :)
ah but those particular compression / write rate stats are compared to mmapv1, yeah