Specifically, I was working on a game inspired by the Bangai-O series by Treasure. Having way too much stuff exploding all at once was always a Treasure trademark, as was massive slowdown, sometimes into the single digits. Oddly, this actually served to modulate the difficulty of such games by slowing down more chaotic scenes.
For my game, things generally stayed at 60fps; at times, though, it would reach as low as 15-20fps, usually for brief periods, but sometimes for up to 30 seconds or so. Mainly due to a custom collision system that involved, at times, a few thousand onscreen actors, sophisticated projectiles with the ability to home and reflect off surfaces, and partially destructible tilemap-based levels.
For some reason, slowdown handled this way always felt really awesome, as opposed to the awful jerkiness of frame-skipping, which always reminded me of trying to watch youtube on a crappy connection.
Aside from asthetics, as this game was as much a puzzle game as an action game, having unreliable collisions was simply not an option.
I still think frame-skip is better for typical games that are trying to maintain a constant framerate, with only brief small dips. However, for games that expect -- or even aim -- to produce sequences of prolonged slowdown, I think a fixed timestep is a more elegant solution.