It was quite an unusual experience. Most interesting was that there was no intermediate state that I was aware of between sleep, and being completely awake and aware of exactly what it was. I'm usually quite slow and befuddled when woken by something unfamiliar, and take a few seconds to figure out who I am, let alone what's woken me. In this case though it was almost like my brain had been puzzling over it while I slept, realised what was going on, and then flipped some internal switch from 'sleep' to 'wake' while at the same time loading a little onStartup message into my consciousness saying 'there is a big spider on your face'. I find myself wondering if there's some evolutionary reason behind it, a little like I've heard there is evidence of an ability to rapidly detect snakes.
On the bright side, I'm never going to worry that spiders might creep over me in the night. If they do, I'll know

*Quite how large is hard to say. At the time it looked huge, but perspective does that. It was large enough to make an audible noise when I grabbed it and threw it at the wall.