It's pretty impressive on the whole. Given a bit of text like the one in a message I got this morning...
The meeting will be on Friday, April 26th at 9:30am in BC 106.
on hover it highlights the date and time bit and give me a little pop-down menu to make an appointment in iCal out of these data:
I can edit the appointment before it gets created, putting it in the right calendar, for example. Great, right? Yes, great.
It also does a few stupid things. First it uses the subject of the message as the appointment text, which would be fine, except the subject very clearly has the same date and time info and it doesn't bother to try to strip that out, even as it removes the leading "Fwd: " from that same text. Second it's apparently too stupid to ever recognize any location info, even when that info is pretty clearly indicated by adjacent and obviously locational prepositional phrases like "in..." or "at...". Sometimes it's going to get those wrong, but a lot of the time it won't, yet it clearly hasn't been written to scrape that info out of a message. Finally this capability isn't active in text that you're typing. So if I'm in Mail, writing a message to confirm an appointment with someone, I can't just click on my own date and time text and make an appointment out of that. I have to go into iCal and do it.
This is one of those insanely great things I love in OS X. It's frustrating when it's senselessly crippled.