I'm running 10.4.1 with PowerMail 5.2 build 4388.
Have not had any problems at all, none, zilch, nada. I did compact and rebuild my database before making a back-up copy and then opening my mail database in the new version. I then immediately compacted and did a low level rebuild, just in case. Again. I've had no issues. I updated to 5.2 for spotlight searching and it works great too.
