By 1971 Diana Ross has her solo career in full swing with four full albums released and string of singles. I'm still waiting was her 6th solo single but in the US made it only to #63. In UK (and Ireland) it was a #1 hit. Ms Ross re-released it in 1990 as a DJ Phil Chill remix and again it charted in UK: #21.
There's also a great 2009 Almighty remix:
Ex-The Supremes Sherrie Payne covered the song in some part of her career, there's a Chinese cover by Faye Wong, and finally Courtney Pine feat Carroll Thompson took the song again to UK charts in 1990 (#66)