I have two power supplies. One looks a little wet inside (electrolytic?) and both have that "Old Mac Smell." Regardless, both seem to boot the IIsi just fine. I did voltage tests on both of them via the Floppy port:

Supply 1 (this is the leaky one)

12v: 12.15

5v: 4.97

Supply 2

12v: 12.18

5v: 4.96

Presently I'm using the second one. My IIsi logic board is also due for a recapping (electrolytic staining is visible), but it's still a fully functional board. I plan on recapping all at once (logic board and one power supply) but it's a big project that I won't do until I need to (and those big caps in the PSU do look threatening! I noticed that using a NuBus card does overwhelm it - my Mac II (toby) video card cannot sustain monitor sync (keeps turning on and off), but that may just be a bad card.