I have the MC6600 and LOVE it. It's a true work horse that just sews without complaints. The built-in upper feed dogs are so useful and you can always flip them up if you don't want to use them (though - why!). I even took my to the school and let the students use it and it never glitched. Love the hum.
So why did I buy another machine? - I wanted a bigger throat space.
Has it been an improvement? - not really. I could shove a lot of quilt through the 6600 and even managed to quilt a king-sized quilt.
Really, $400 seems a reasonable price. I take mine for servicing every year and it has never needed anything beyond cleaning. Use it a lot because that's what it likes. Usage keeps everything well oiled.