Well, 'poorly' as in it didn't crack the top 20 as expected (peaked at #29, even though it made it to #1 R&B). I've read in a book that Otis said it lacked a certain edge or something, but if I didn't know any better and you played me that song, I would've thought it was a smash pop hit. It's fast paced and catchy, and I think that the lyrics were indeed more 'direct' than some of the others Smokey wrote. But the general public didn't seem to feel the same.
It's not all bad of course as we got Ain't Too Proud To Beg as the next single and Norman Whitfield's sure hand for the next few years, but I've always wondered exactly why Get Ready under-performed. Ironically it's now seem to become one of the group's most famous songs and they even performed it with Eddie Kendricks in 1971 on the Ed Sullivan Show, not to mention all of the cover versions and Rare Earth managing to make it a huge hit later on.