Fair amount of commanality, but there is much greater difference between MD82/83 and MD90 cockpits and systems than between there is between 767 and 757 (step up or step down to enter).
MD90 performance is significantly improved over the MD82/83. V2500 engines provide much greater power with considerably smaller noise footprint.
Only major drawback I've seen with AA's MD90s is the VSCF (variable speed constant frequency) electrical generation system. Too complex and unforgiving of off-frequency variations that causes many of the computer/electronic systems to fail self-tests. That's why one of the main ways to fix the MD90 is to power down the entire aircraft and start up again (i.e. reboot).