(I think I'm in the right forum now)

I've read many of the threads here dealing with the mass air sensor (same as air mass meter?) and I'm not sure if I should have it replaced, replace it myself (how hard is it?) or just live with the check engine message. It's a '97 E320, all functions work, drives fine, mpg's same as new; but a week ago "CHECK ENGINE ENGINE ELECTRONICS" popped up on digital display on dash. My dealer mechanic said probably air mass sensor needs replace, at $600.00 cost, but he reset computer to eliminate message and said wait and see if message comes back. It came back yesterday. So, should I try to replace it, should I let dealer replace it, or should I do nothing until car starts running erratically? What kind of erratic behavior will I likely see if I don't replace it?
Thanks to MBDOC for his quick reply. I checked fastlane; the part is $275. I think I'll try replacing it myself.....what does the MAS look like and where is it?