In a word: FAIL.
While the cakes puffed up somewhat nicely, and burnished to a nice golden-brown on top, they were fused to the muffin tins like someone had mortared them in there. I had considered greasing the tins, and/or using liners, but there was enough butter in the batter (ha!) that I thought I'd be OK. Well, I thought wrong.
It took me the better part of an hour to remove, chunk by chunk, the weird little puffs from each well, and then soak and scrape and soak and scrape, repeat x3, until my muffin tins were clean. My frugal little heart wept: all that batter, wasted!
Lesson learned: no matter how much butter is in the batter (ha!), grease the dang pan.