I recently got the incra ibox, and since it was mentioned in this thread, I'll throw in my experiences with it.
It has it's own miter slide, which allows for fine tuning to get a "perfect" fit. The adjustment is tight so I had a hard time getting it just right. It was either a little too loose or a little too tight. Once I finally got it where I wanted it, it worked fine. Like the Rockler jig, the jig screws to this miter slide and you have to make sure it's square or problems will ensue.
Like the Rockler jig, the ibox has a backing plate to prevent chip out. It's MDF though, not plastic. I don't know if this is a good thing or a bad thing. If you always use the same size blade, you can keep the backing in the same place for subsequent cuts and I don't think you'll get any chip out. Obviously, if you alter the size of your fingers, you'll need a fresh backing for a good fit.
The included DVD makes setting up look like a 30 second job. Well it's not. I followed the directions on the DVD exactly and my joint was a bit loose. The fingers should hold against gravity, but not if you pull it apart right? I took a few minutes playing with the jig to figure out exactly what moves with a turn of each knob. Once I did that, I figured out how to alter the size of fingers to exactly what I wanted. Now the fingers are snug enough to not fall apart, but not so tight as to require excessive force to get it together.
At the end of the day, the jig works with no issues. There was a little more set up time than I expected, but it's not too bad. Just have a scrap piece or two to try out a test fit before running your good pieces through.