I'm running Vista 64-bit and have yet to see the memory leak (system has been running for over a week without a restart, though it does sleep at night). I am noticing an odd inconsistency inwhich the title bar buttons (and context menu, sadly) appear/disappear in certain programs. However, as tranglos notes, this affects apps that also add buttons to the title bar (DOpus 9, for example), so I'm living with it.
In sum, version 5.4 is exceedingly well behaved on my system and uses about a third the resources that UltraMon does.