I've decided to investigate several bands/artists myself and determine whether what last.fm gives as similar artists work out or are off. Today, it's the Red Hot Chili Peppers.
The Red Hot Chili Peppers are classified as mainly alternative rock...though that genre is broad. Styles explored by the band include funk rock, pop rock, funk (specifically freaky styley, funk metal (earlier albums), and sometimes, rap rock (aka, rap-like vocals) and psychedelia influences, but rarely.
So, let's see what last.fm considers similar. I have filtered out the first 50 bands/artists that showed up as similar on 6/1/2014. And I made comments under each of the bands/artists based on what I know.
--Well, he's the guitarist of RHCP...but I don't think his music is similar to RHCP.
John Frusciante and Josh Klinghoffer
--Well, more RHCP side-projects. But once again, I have no idea if they are similar