From the moment I first heard this around 1980, I loved this song. It is possibly my favourite Marvin Gaye song. I've always been hooked on Marvin Gaye's vocal control. Even to my 10 year old ears he didn't sound quite the same as anybody else. Gaye was actually a drummer before he was a singer and his sense of rhythm and timing is impeccably precise. This song is a nice example of both that and his flexibility at improvising around a melody. It's not a song people would pull out - I don't actually know many people who do know it - but it's a perfect balance of control, skill and emotion. Coming from 1968 it sits between his earlier romantic songs and the later socially aware material forming a funkier, edgier bridge between the two. Typical Motown, the backing singers include The Originals so I guess they happened to be around free at the time. I love everything about it.
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Those of you digging on Apple's "Jason's Verse" spot about the Slow Roll Detroit movement will note that this is the Marvin tune that Amerigo Gazaway fused into the remix of Mos Def's "Undeniable," used in that commercial.