My memory is a little sketchy, but the way these episodes have been playing out, this is the first time Supernatural will have four continuous episodes, beginning from the sixth season finale. While I don't think this serialization will last too much longer, it's interesting to see the writers play around with the cliffhangers. It adds a sense of doom, as the Winchesters went from dealing to crazed Castiel to Leviathan to Lucifer in Sam's head in quick succession.
The episode ends on a bold note--Bobby possibly dead, Sam seizing, and both Sam and Dean heading to the hospital where the Leviathans are having a party. Yeah, pretty terrible circumstances.
The Leviathans are dangerous and evil enough, but they're pretty generic adversaries. We've already see creatures inhabiting bodies plenty of times in the past and the black stuff easily recalls black oil from The X-Files. I'd like to see more originality from the writers regarding the Leviathans.