I am also experiencing problems. For example, I'm currently subscribed to the "Happy Singles Thread". I visited it some time yesterday evening, since when there have apparently been a number of new posts; I have yet to receive notification of them in my email.
This isn't an isolated incident - I'm routinely finding that either emails don't arrive at all, or else several turn up all at the same time, several hours late in most cases.
I have also noticed another glitch: in theory, I should only receive one notification email per thread until I next visit that thread. Instead, I've been getting an email for pretty much each new post even though I haven't had the time to visit it for a couple of days. When they arrive at all, that is ;-)
Is this just me, or does anyone else have this? All these problems began after the upgrade to vB3 IIRC...