It's a matter of taste and personal preference.
I am a fan of chrome...as long as the enhancements are subtle. Too much chrome can look gaudy.
Consider that in order for the "look" to work, it must complement other chrome features of the car. So if there is little or no chrome elsewhere, then the mirror covers would look out of place.
Earlier MBs had more liberal use of exterior chrome, but newer designs shunned much of the exterior flair in favor of cleaner, more streamlined profiles.
That being said, I like the chrome mirror covers...I plan to get a pair for my W124...and I have a different styling approach for that car than say, for my SL.
Many here wouldn't consider chrome wheels let alone chrome mirror covers...so I'll bet you'll get replies telling you otherwise.
Chrome stands out the most on vehicles with dark finishes. It draws the least attention on silver vehicles...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car