We are seeing lag, but it is not so bad as to stop our raid. Our server often has a queue to log on during our raid times too.
It sucks though. Here's hoping for a fix soon. Nightmare (when we get it) will truly be a nightmare with the lag.
It was so bad on Tuesday on Stormcaller, we actually halted raid, went outside and spent 20 mins forcing ourselves into another instance server. Then recleared trash and killed the rest of the encounters with less lag and less problems related to lag.