In general, V5.01 is well behaved and I have not encountered major bugs but there are a few areas that could be improved. I am running under Win7 x64.
1) When you launch the program it does not always open as a full window; sometimes it just appears as an icon in the task bar.
2) The legend in the RT60 tab is not always updated: sometimes the name of the previous measurement will remain instead of the latest measurement. Sometimes different curves like EDT, T20, or T30 will appear even though they are not selected.
3) The biggest problem for me, right now, is a bug that prevents taking successive measurements using a TASCAM US-144 MKII. The problem may be in the Tascam ASIO driver but I cannot take more than one measurement at a time (I take 4 samples per measurement). I have found that saving the data and restarting REW is faster than the suggested procedure of switching to the Java driver and then back to ASIO. Incidentallly, if I cancel the measurement, even during the fourth sweep, the buffers are cleared and it is possible to take another series of measurements.