Its not bad. Opportunist burglary is the most common crime, quite a few students get done when they've had house parties and invited randoms in off the street, or when they've left windows open and had playstations/ipods etc disappear out of their room.
Re violent crime it's not bad. Headingley and Hyde Park and the city centre are pretty safe. Woodhouse, Little London a bit more dodgy but unless you get a house there you'd never need to go there. If you buy drugs then you increase the risks slightly because you start to have contact with dodgier people.