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I have this one. I found it extremely helpful in the pull-up range you are in. Remember, there is a reason you stay in the 8-10 rep range for most of the exercises. 1-2 good, but 6-10 is better. More importantly (for me at least) was that it let me do more than one (or zero!) of the more challenging pull-ups to start with, which works some different muscles.
I found it far better than the chair, as I could be much more consistent in the amount of assistance I received. The down side to them is that they do change the resistance curve of pull ups - you get a lot more "assist" at the bottom than the top, so you have to be careful to not "launch" yourself. This isn't much different than not "cheating" by rocking your body during curls and such, though, so it's not a huge deal - it's just another place where form matters.