I disagree with the previous posters. Forget the assisted pull-up and dip machines. Do the actual exercises first in the workout, when you are strongest, and if you are too weak to do a single rep in each, do negatives. Jump to the top position and lower yourself as slowly as you can. This should build your strength nicely. If you are strong enough to do about 3 pull-ups, do sets of 2. About 4-5 sets should do the trick. When you are strong enough to do 5 or so, do ladders. This technique means to do one rep, rest, do two reps, rest, do 3, etc, until you hit your max, then drop to 1 again and move up again. It is psychologically quite easy in the early sets, and a good way to get a good amount of reps in. I have had success this year coaching athletes using these methods, including some fat rugby players who couldn’t do a single chin up in April, but now are approaching ten.[/quote]
The problems with assisted machines are that (a) your posture is off and (b) people tend to go too easy because they can.
These are the similar problems with pulldown machines. Yet I would argue that both could help you get to the point where you can do a pullup. After that, they’re practically worthless.
Negatives sound like another good alternative.