Oh ya it's definitely real, also definitely exaggerated/exploited to sell units. We have had days in the rec studio where the the signal is clearly different, sometimes audibly "dirty", other times the signal wasn't as full/strong. I'm talking a one day difference where we shut the amps down, didn't move a single knob/connection, locked the door, and came back the next day to a different guitar tone, vocal signal, ext ext. In a studio situation where you have the ability to play tracks from day 1 and day 2 back to back it is easy to hear a difference. Granted a lot of factors in play here, but the engineer will literally call and say no-go today due to dirty power.
Also in my own home (located next to a large transformer) I have integrated a tripp-lite medical grade power conditioner/noise suppressor and it cut down on hum in amps with a lot of pre-amp gain.
My house was also built in 1910, i'm sure more modern electrical systems are better and solve some of these issues with better shielding and such.