Here's what works for me... If you want to do 20 consecutive pull ups, what I'd recommend doing is as many pull ups as you can consecutively, wait later on in the day and do more till failure until you reach 20 pull ups. Record the time taken and start again. Eventually you'll get to the point where...

I think the main problem with ab wheels is when you use them exclusively for working your abs without focusing on other muscles. If you're doing push ups, chin ups, dips, squats and lunges and using the ab wheel then it'll definitely compliment what you're doing.