I can settle for 30wpc, but why bother when 120 or more can be had for so little (with a killer 4-gang tuner and versatile preamp functions and such)? There is something to be said for a power amp that doesn't even have to try to crank out your tunes.
To directly address the question, I use 89db/w sensitive speakers, and the 28wpc Kenwood I was using did just fine. I couldn't DJ parties, but I've got the G-8700 for that, so it's all good.