Currently got a JMJ (just picked up from Will off this fair site). First impressions are that it has a very nice warm but punchy core tone with the stock Fender flats. It has that very played in feel that comes with the MIM roadworn territory, I think the attention to detail on the relicing is a notch up on the Jazz roadworns I've played though (e.g. very nice checking on the body).
Compared to my CIJ Mustang made in 2002 (I think it's a 1966 re-issue?) the neck is a bit chunkier (more playable to me) and the headstock a little thicker. The JMJ has a satin finish neck, the CIJ a vintage tint gloss. They both have SD pickups which are more similar than different, the JMJ seems a bit more 'vintage' but it's hard to tell unless they were wearing the same strings (my CIJ has Fender rounds).
At the minute I'm struggling to justify having 2 similar Mustangs so my CIJ is up for sale.....however I'm secretly hoping SWMBO forgets my promise to sell one. You wouldn't go wrong with either the JMJ or the CIJ; In real terms, if buying used there's probably £200 between the 2 only you can decide if the difference is worth it to you.