Unfortunately it's a little hit or miss. In fact, rather bizarrely, it's really good at finding random times or dates strewn haphazardly around in an email, but when someone bothers to put together a nicely formatted date and time - say like this: Wednesday, 7/23, 3 PM-4:30 PM - Google can't seem to figure out that might be something I want to add to my calendar. I'm probably doing some bad estimation here, but I'd think that that properly formatted shit is way more likely to actually be a calenderable thing. (Calendarable - is that a word? Whatever, it is now.)
The old idiom something is better than nothing doesn't apply to stuff like this. The fact that it works sometimes and not others just creates frustration when it doesn't and the user is expecting it to. If it never did it, that'd be better all around. For some people it might be annoying, but they'd quickly learn not to expect it; this is not a good user experience for them, but it's certainly a better user experience than what we've got now. For others... they may be very used to manually jotting down dates and might never miss the feature at all if it wasn't there.
By the by, Remember the Milk probably has one of the better time/date free text parsers I've ever used. It can handle most anything and it was built by a tiny little team. If they can do it, Google can do it.
