I use the P90x pull up assist band from Beach Body. Way better than using a chair, and lets me pound out pull up reps the entire time.

So far (I'm in recovery week of Phase 1) I've went from being able to do just 2 pull ups unassisted to being able to do 4-5.

The one you linked is 10 dollars cheaper, but doesn't have the clip to the bar. The clip is nice, as it is easy to put up and take down the assist.