I just put one on my IS.... The one they recommended when I called was the H15400. What Joe said about the bags spreading is correct. It makes them spread out a bit. If you don't extend the cross bar underneath they kind of form a 'V'. I really need to do what Joe did and split my cross bar and add a piece in to have them straighten out a bit.
Other than that, the hitch works great!!. Pulled it to Galveston earlier this month and I had not trouble with the hitch at all...
I would recommend having 2 people present when installing it. I did mine by myself. Wasn't impossible, but 4 hands are better than two for sure..

Cheers,
Robert C.