Product review (5 stars): Love it!
I actually bought 2 of these, and couldn't be happier!

They fit my Artisan 5qt glass bowl perfectly - even the pour spout!  There's no gaps, no leaks.  its easy to clean.  It works great for covering the bowl when in the fridge.  It works even better for covering the bowl when its on my counter - it keeps dust, etc out.  The depression in the middle allows for the tilt head to be down and locked when not in use.

The cover is made from a firm but, still flexible silicone, in an attractive gray color that won't detract from whatever color you chose for your mixer.  It can be hand-washed, or tossed in the dishwasher.  It says its freezer-safe, but I haven't tested that yet.  It works fine in the fridge.  I wouldn't use it in an oven or microwave, nor a stove top, especially since the bowl itself is not designed for such.

Its a shame that one doesn't come standard with the mixer.