I'll put this out there first of all: I don't really like bad boys, in real life or in books.
In real life, because I was born without a rebellious streak, I guess. In books, because I hate to see a bad boy gone soft.
Have you noticed this? Hot Guy is mysterious, moody, dark. Girl is drawn to him because he just feels so deliciously dangerous. There is tension. This is good. I'm totally on board up to this point.
Tension breaks. They get together. Bad Boy starts sharing his feelings like a freaking girl! And he's nice and sensitive and kind and patient. And it seriously bugs me. This is not the same guy Girl was initially attracted to. He has become her, with a deeper voice. Guh.
I say, if he's a Bad Boy, he has to have a reason to be bad. And he has to stay bad. Now, I'm not saying he should slap her around or forbid her from talking to her friends or anything. And of course, a guy would open up more to someone he loved than to the general public. But please, don't make the Bad Boy a pushover as soon as he falls in love.
I'll give you one example of Bad Boy done right: Jace Wayland in the Mortal Instruments series by Cassandra Clare. (No spoilers, guys, I just finished Book 4) Jace is arrogant, sarcastic, and can be downright ruthless with his cutting words. He is emotionally cut off and aloof. He falls for Clary and--wonder of wonders--is still the same guy. Yes, he opens up a bit to Clary, she can see how his sarcasm is a coping device. But he still does it. He still annoys her. She loves him anyway, and I totally buy it because that's who she fell in love with in the first place.
So, Bad Boys. Be nice to the girl you like, but please, continue to be a jerk for your unique reasons, and I will appreciate you all the more for it.