I think every kid is different and goes at their own pace. The most important things I learned about potty training are you must wait until the child is physically ready and pull-ups are for Mommy!
My oldest ds was all set at 3 1/2 and it seemed like it took forever. We used a seat that fit on the toilet -- I didn't want to use a potty chair, because I only wanted to show him where to go once..and I didn't want to clean it or have it carried around. I also gave incentives -- m&ms for #1 and a dum dum lollipop for #2. I also set the timer for every 20 min or so to go potty. I think what really helped my son was going to pre-school 3 mornings a week and see that everyone else was potty trained while he had pull ups on and a spare in his back pack. I was also using pull ups forever, but realized they were more for me than him. Once you go to underwear, children seem to immediately understand cause and effect!
As a bonus, my other ds, who was 2 1/2, just watched big brother and basically trained himself, so I got a 2 for 1.