I upgraded to 5.4 yesterday on my Win 7 64 bit. Lr 5.4 wouldn't open, showing a dialog that said "assertion failed". I googled the error along with Lightroom 5.4 and found quite a lot of chatter about it. None of the fixes recommended worked. One suggestion was to run Lr "As Administrator", the other was to remove the Preferences file and let Lr rebuild it. Neither worked.
I went back to Lr 5.3 and it works fine.
I searched the error here and haven't found any comments about it, at least not recently. It appears to be primarily Win 7/64 users that are getting the error. Has anyone else experienced it?