http://www.rowanpix.com/reenactments/images/a-Knight-Crossbow.jpg
How do people use crossbows? As in how does it compliment their build?
I use crossbows with heavy armor. Bows are quicker, to use, they are silent and compliment a lighter stealthy character whereas a crossbow is very slow and noisy (gives away your position if used in stealth mode) which to me is better for a well equipped warrior / knight style of play. It gives the warrior ranged attack if needed and at closer range it gives a hard couple of strikes before the enemy is upon you at which a well armed warrior can switch to melee combat to finish the job.
Crossbows come in Steel or Dwarven which are both materials used to make heavy armor so match nicely depending on armor worn (shame Tamriel weapon smiths haven't upgraded the steel with quicksilver or ebony).