I'd be filled with awe, wonder, amazement, dread, depression, and absolute despair. Waking up in the Half-Life universe would be AWESOME, what with the cool sci-fi tech (gravity gun, portal gun, etc, etc), but seeing as the proverbial "shit" is always hitting the fan in that universe, and it seems like the rest of the universe is either out to get us or is playing the role of Finland, it wouldn't be a nice place to stay.
Plus, you know, the enslavement thing; the zombification thing; the "theft, rape, and death" of everything around you thing; and the whole infertility thing.....not fun.
'Course, getting to pal around with the likes of DOG, Alyx, Barney, Eli, and Issac would be at least one up side.