Badmouth-
Not sure which you're riding, but I've got the ventura grabrail/rack, and a Cortech tailbag, they fit well together. Though. I have the tailbag on the seat, butted against the grab rail, not out /on/ the rack. . .not sure which place you were looking for comments on. Mine is on a TBS. The rack actually gives some good locations for the bungees on the Cortech bag to attach to.
you can see the setup here:
and a little closer up from the back to see the rail/rack- check this out:
http://farm2.static.flickr.com/1167/...b0d64bc8_b.jpg
(too large to link politely.

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