AFAIK, it works like everything else in synchronous mode, e.g. the transaction won't commit until the data has reached the secondary, this could obviously lead to some slow down with large files (it's the price you pay for synchronous).

Also, make sure you're at least on SP1 as there was a serious bug affecting failover times in RTM.

This functionality is all pretty new and there isn't a lot of data on how this performs in the wild yet, so I'd plan some time for thorough performance testing before committing.