The problem with pull-ups is that they work too darn good. When a kid wets himself, there's no physical consequence (its all absorbed)! Also, pull ups are EXPENSIVE.
Every child and family is different. My family customs don't include pull-ups. In the old days, there were no pull-ups, so kids went cold turkey. I think there's some credit to this, because there's no confusion in the kid's mind. A pull-up looks and feels like a diaper, so I can't blame a child for "using" it. Heck, some days I wish could wear a pull-up the way I run around!
It's easiest to diaper-wean in warmer weather because there's a lot less clothes to deal with. It also helps if your son has preschool friends/neighbors/cousins who are also potty-training at the same time. (Peer pressure works--use it!)
When going straight to underwear there will be couple accidents, When I trained my son, if an accident happened, we just calmly cleaned it up, saying, "I know you'll do better next time!" (No punishment.) Another good attitude is simply stating, "Everyone uses the toilet. This is simply what we do."