Melodyne plug is faster and easier than auto-tune but after years of using it, I can't stand what it does to the vocal anymore. Totally messes up the frequency response not to mention the timing which can get screwy sometimes. I've been using graph auto-tune more now, and it sounds so much better...
Anyways a tip for bad singers... get as close as you can as fast as you can because they're not going to get better with more takes. Something that works well if you're not gonna do extensive tuning is to get them to double all their vocals. It has a way of hiding the out-of-tune-ness.. must be a psychoacoustic thing. When you have 2 takes that are out of tune, the line becomes blurry and the brain will pick out the average in-between note rather than hearing the two individual tracks.