No personal experience with people telling me X would look good (I'm allowing for good friends, as well as SO's), but most of what I do/wear is based on it being comfortable (which I why I don't wear jeans - I don't find denim comfortable), simple (which is why I cut my hair - I used to wear it down to my shoulders, but it tangled a lot and was a pain to deal with), and shirts that fit my general style (chemistry and snarky). If someone who I cared about wanted me to try a different style that wouldn't require a complete and total wardrobe change (I can't afford to do that), I'd be fine with trying it.
In terms of what I'd ask of an SO/close friend (and partly triggered by Glass Mouse's post)... if I actually thought I had any ideas worth mentioning in any kind of serious way (unlikely; I don't really pay attention to fashion), I'd probably mention it, probably in a "hey, have you ever considered X? I think it might look good on you" way. Also, it certainly wouldn't include any of the "girlifying" suggestions, since I don't find those particularly attractive (well, beyond being eye candy, sometimes; if they decide to dress like skanks, I'm willing to be a bit of a non-obvious pig).