5'10 with a 31" inseam here and was having a little bit of knee pain with the 172.5s that come on most bikes my size. Swapping to 170 seems to have alleviated the problem. I was even considering 165 before that, but didn't want to lose too much leverage for climbing. Other than the happier knees though, I really don't notice the difference.
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It's like riding a bicycle