It used to take a lot of takes but now-a-days with digital editing it's easy to slide it around so it matches.
ADR is not as hard as people think it is - unless you don't have a musical ear or a sense of pitch and rhythm.
All you are doing is listening to what you said before, and then you say it again. It's like repeating yourself. It's simple. But some people who are tone-deaf and have no sense of timing or rhythm, you can spend hours on one line. Trust me, I've recorded actors like that before.
You do have to match the lips - that's the point. Sadly, some people don't do it as well so it looks bad - some movies where you go "that didn't look right" on the sync is usually the case of bad ADR editing. Law Abiding Citizen I heard was an example of bad ADR.
ADR is not always necessary. In fact, it's a sign that your set sound team sucks and didn't do it right the first time.
The Hurt Locker is a great example of this. Plus, I know absolutely for a fact that it wouldn't have won Best Picture if they had to ADR the whole movie - so the production mixer should be given all of the kudos in my opinion. Without the original performances it would have suffered greatly as a movie.
That's another reason why capturing sound on set the first time correctly is so important.