I haven't experienced IB J, but I asked this same question for a trip from LAX-CDG that I booked a few months ago. The advice I received was that that AA J and IB J are about the same--nothing special. Decent hard product, variable soft product ranging from slightly below average to pretty decent. The advice was to go with the best price/sked.
Secondhand...take it for what it's worth, which may be nothing.
I ended up booking IB J just to experience it, but then a J seat opened on AA 108 from LAX-LHR and I jumped on it. Very glad I did...saved $100 fuel surcharge!
Here's a link to that conversation:
http://www.flyertalk.com/forum/ameri...l#post26954259