just to say i have both rackets. i used JJS for about 3 seasons now and love the racket. Changed from BS lyd and just love the more solid feel of this racket and the extra power if gives over the BS series whilst only losing a little bit of speed in comparison.

I also got the MX 90 wondering if it would be the upgrade to the JJS. to my surprise i found it a little stiffer and much less forgiving. whereas i felt the JJS generated a lot of power easily, i found the MX90 required much better timing.
As i was just coming back from a tennis elbow injury i found it quite hard to use.

However before my tennis elbow injury i found it was VERY rewarding when you get your timing right and offered a noticeably more powerful smash. Drives were also sharper and the racket provided excellent touch and feel.

In conclusion i think the MX 90 would be an excellent replacement to the JJS provided you tune your technique prior to switching rackets. But i would say the MX 90 certainly suits the power player. (perfect for me as i am a super aggressive doubles player - Gentleman off the court obviously)